1 Introduction

Soil salinization and alkalization pose significant ecological challenges, particularly in the northeastern region of China, where the Songnen Plain spans 17.0 million hectares. Approximately 20.12% of this area, equivalent to around 3.42 million hectares, consists of saline-alkali lands (Chi et al. 2012). The salt components found in these soils are primarily Na2CO3 and NaHCO3. Li et al. (2006) reported that the saline-alkali soils of this area exhibited distinctive characteristics such as elevated levels of Na+, sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP), and pH in the topsoil. The increased Na+ levels in this soil type can disrupt soil aggregates through processes such as slaking, swelling, and clay dispersion. This adversely affects the infiltration capacity and hydraulic conductivity of the soil, resulting in poor physical characteristics (Morsy et al. 2022; Oster et al. 1999). Furthermore, excessive Na+ in saline-alkali soils can affect nutrient availability and absorption by direct ion competition and, indirectly, by decreasing the mass flow of nutrients to roots due to increasing the osmotic pressure of soil solution (Rengasamy 2010). Saline-alkali soils of the Songnen Plain typically exhibit low levels of nitrogen, phosphorus and potassium, which is attributed to limited organic matter infusion from crop biomass (Chi et al. 2012; Lakhdar et al. 2009), significant organic matter loss (Li et al. 2006; Liu et al. 2017a), and unfavorable physical, chemical and biological properties (Gong et al. 2021). In addition, the wetting of these soils can cause the dispersion of aggregates exposing shielded organic matter to accelerated decomposition (Qadir et al. 1997). The detrimental impacts of high salinity and sodicity on soil microbial populations and enzyme activity indirectly influence soil nutrient transformation and availability (Shi et al. 2019). These factors contribute to soil structure deterioration, impaired soil hydraulic characteristics, nutrient imbalances, and limited water permeability, significantly restricting agricultural production in this soil type (Kamran et al. 2021).

Addressing global food security challenges necessitates the development of strategies for cultivating unproductive salt-affected soils (Wang et al. 2023a). For instance, rice cultivation can efficiently mitigate the adverse impacts of compromised soil structure, while facilitating the gradual leaching of salts from the topsoil through irrigation. Therefore, employing low-salinity water for rice cultivation emerges as a successful method for rehabilitating saline-alkali soils and ensuring food security (Chi et al. 2012). However, the changing global climate poses a serious threat to rice production in saline-alkali lands due to freshwater scarcity, increasing soil salinization, and soil nutrient depletion (Ran et al. 2019; Wang et al. 2009). Novel technologies or sustainable practices are urgently required to rehabilitate deteriorated saline-alkali soils, reclaim soil structure, improve nutrient availability, and mitigate the impact of climate change on the soil production of saline-alkali ecosystems.

Due to its various benefits, biochar holds promising potential as a soil amendment for the revitalization of salt-affected soils (Saifullah et al. 2018; Wang et al. 2023a). Recent studies suggest that adding biochar can enhance soil productivity and reduce saline-alkali stress in salt-affected soils by increasing soil organic carbon (SOC), nutrient bioavailability, cation exchange capacity (CEC), and specific surface area of soil particles, as well as stabilizing soil structure (Joseph et al. 2021; Nan et al. 2022; Saifullah et al. 2018). Furthermore, biochar holds the potential to serve as a habitat for soil microorganisms and enhance their activity, indirectly improving the nutrient status of salt-affected soils (Cui et al. 2021; Palansooriya et al. 2019; Wang et al. 2023b). Our prior study also demonstrated that biochar, especially when combined with nitrogen fertilizers, could significantly reduce the sodium ion concentration in the root zone soils, mitigate Na+ buildup within rice plants, ameliorate the saline-sodic stress on rice, and enhance rice yields (Piao et al. 2022; Ran et al. 2019; Yao et al. 2021). However, the impact of biochar application on crop growth may not always yield positive responses due to factors such as: (i) increased N losses as NH3 to the atmosphere from soils because of increased soil pH (Sun et al. 2017); (ii) nutrient imbalances and N-immobilization due to microbial activities (Borchard et al. 2014; Werner et al. 2018); (iii) effects on the balance between vegetative and reproductive growth (Vaccari et al. 2015); and (iv) the presence of potentially phytotoxic compounds from biochar (Saifullah et al. 2018; Zhang et al. 2016). The diverse reactions of crop growth to biochar can primarily be attributed to soil types, plant varieties, and biochar types (Gunarathne et al. 2020; Saifullah et al. 2018), and their complex interactions (Wang et al. 2023a; Xu et al. 2023). Previous studies predominantly focused on upland crops or above-ground components in short-term trials. These studies were mainly conducted using small containers in laboratory settings, greenhouses, or through short-term field trials (1–3 years) using a continuous application pattern (Lashari et al. 2015; Liu et al. 2022; Santos et al. 2021; Wang et al. 2023b; Zhang et al. 2020). Limited data support the long-term impacts of the one-off application of biochar on soil physical and chemical traits, salt concentration, nutrient availability, soil enzyme activities, and rice yield in severely saline-alkali paddy soils. In the Songnen Plain, our previous studies indicated that the application of biochar into saline-alkali paddy fields may enhance soil nutrient status, organic matter content, C/N ratio, enzyme activities (Yao et al. 2021), while reducing soil ion toxicity and osmotic stress in a 2-year trial (Li et al. 2022a; Ran et al. 2019). However, uncertainty remains regarding the long-term influence of biochar on the functionality of highly saline-alkali paddy soils and their primary productivity.

Therefore, a 6-year field experiment was conducted in a highly saline-alkali paddy field to evaluate the long-term effects of a one-off application of biochar at four different rates, with or without the addition of nitrogen fertilizer, on rice yield and related soil properties. This study aimed to (i) demonstrate the dynamic changes in salinity and alkalinity after 6 years of the one-off application of biochar into highly saline-alkali paddy soils; (ii) evaluate the long-term and sustainable effects on physicochemical properties, nutrient bioavailability, and enzyme activity of highly saline-alkali paddy soils; and (iii) elucidate the mechanisms by which the one-off biochar application regulated the soil health and production ability of highly saline-alkali paddy soils.

2 Materials and methods

2.1 Experimental site

The field experiment was conducted at the Saline-Alkali Research Base of Jilin Agriculture University, located in Da’an City, Jilin Province, China (45°35′N, 123°50′E), from 2017 to 2022. The experimental location experienced an average annual precipitation of 415.49 mm, evaporation of 1697.11 mm, and an average temperature of 4.69 °C. Based on the International Society of Soil Science classification, the soil was categorized as saline-alkali soil with a clay loam texture. The basic physical and chemical properties of the soil used in this study are detailed in Table 1. The soil texture consisted of 23.13% sand, 38.14% silt, and 37.60% clay. The physical and chemical properties of the soil in the 0–20 cm topsoil layer were as follows: soil bulk density (BD) was 1.61 g cm−3, the electrical conductivity of soil saturation extract (ECe) was 24.08 dS m−1, pH was 10.10, sodium adsorption ratio of soil saturation extract (SARe) was 368.11 (mmolc L−1)1/2, exchangeable sodium percentage (ESP) was 71.11%, cation exchange capacity (CEC) was 10.99 cmol kg−1, soil organic matter (SOM) was 0.64%, total nitrogen (TN) was 0.27 g kg−1, alkali-hydrolysable nitrogen (AN) was 16.30 mg kg−1, available phosphorus content (AP) was 9.13 mg kg−1, and available potassium (AK) was 107.25 mg kg−1.

Table 1 Physicochemical properties of the soil
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2.2 Experimental design and field management

The field trial employed a split-plot design, where the main plots were designated by nitrogen levels and the subplots were determined by biochar application rates. The two nitrogen levels encompassed no nitrogen application (N0) and the amount of nitrogen applied by local farmers (N225). The study comprised four distinct biochar application rates: T0 (no biochar), T1.5 (1.5% biochar, w/w), T3.0 (3.0% biochar, w/w), and T4.5 (4.5% biochar, w/w). The field application rates of the four biochar treatments were 0, 20.25, 40.50, and 60.75 t ha−1, respectively. Three replicates were conducted for each treatment. A total of 24 plots measuring 6 × 5 m were established in April 2017 for the field experiment. Buffer zones of 0.5 m in width separated each experimental plot, and each plot had its dedicated irrigation and drainage system. The experimental fields had a prior history of rice cultivation spanning 2 years. The Japonica rice (Oryza sativa L.) variety, Changbai 9, was seeded in the greenhouse during mid-April. Then, the seedlings were transplanted into experimental plots with a row spacing of 0.3 m and a planting spacing of 0.2 m in mid-May each year. Each hill consisted of three seedlings. The rice was harvested during the first 10-day period of October each year. For the N225 treatment, nitrogen fertilizer was administered at a rate of 225 kg N per hectare, applied with 50% as basal fertilizer (urea, 46% N), 20% applied at 10 days after rice seedling transplantation (ammonium sulfate, 21% N), and 30% added in the early flowering stage (urea, 46% N). The phosphate fertilizer (triple superphosphate, 50% P2O5) was added as the basal fertilizer at a rate of 75 kg P2O5 per hectare. The potassium fertilizer (potassium sulfate, 40% K2O) was used at a rate of 100 kg K2O per hectare, with 60% applied as basal fertilizer and 40% applied in the early flowering stage. In the N0 treatment, no additional nitrogen was applied (0 kg N per hectare), but additional amounts of 75 kg P2O5 per hectare and 100 kg K2O per hectare were administered. Other field management practices were similar to local production practices.

The biochar used in this experiment was obtained from peanut shells through a pyrolysis process conducted without oxygen at 450 °C for 4 h. The biochar was procured from Liaoning Jinhefu Agricultural Development Co. Ltd. Its physical and chemical properties were as follows: pH was 7.94, cation exchange capacity (CEC) was 78.69 cmol kg−1, surface area was 7.41 m2 g−1, electrical conductivity (EC) was 7.88 dS m−1, carbon content was 540.64 mg g−1, nitrogen concentration was 15.93 mg g−1, phosphorus concentration was 0.74 mg g−1, potassium concentration was 12.53 mg g−1, calcium concentration was 2.01 mg g−1, sodium concentration was 1.17 mg g−1,and magnesium concentration was 0.25 mg g−1. In Spring 2017, a single application of biochar was applied to the experimental plots. It was meticulously incorporated into the uppermost soil layer (depth ranging from 0 to 20 cm) using a wooden rake.

2.3 Sampling and measurements

Refer to the Additional file 1 for specific sampling and measurement methods used in this study.

2.4 Statistical analysis

The significant differences in the measured indexes among biochar treatments were examined by using one-way analysis of variance (ANOVA). The interaction effect among biochar, nitrogen, and year was evaluated using three-way ANOVA. t tests were performed to compare nitrogen factors. Statistical analysis was conducted using SPSS 18.0 software (IBM Corp., USA) and the least significant difference test (LSD) was applied. Origin Pro 2023 (OriginLab Inc., USA) was used for principal component analysis (PCA), Pearson correlation analysis, and generation of figures.

3 Results

3.1 Physical properties of soil

3.1.1 Soil bulk density (BD), total porosity (Tp), and saturated hydraulic conductivity (Ks)

ANOVA revealed significant effects of biochar treatment, nitrogen level, year, and the interaction of biochar treatment and year on soil BD (Fig. 1A). BD significantly decreased with an increase in the biochar application rate in all 6 years of the study. Compared with T0, the average BD under T1.5, T3.0, and T4.5 treatments significantly decreased by 11.21%, 16.33%, and 25.57% (Fig. 1A), respectively. BD was significantly different among all biochar treatments in 2021 and 2022. With the increase in the years of biochar application, BD exhibited a decreasing trend with each successive year. The average BD value was 8.09% and 8.91% lower in 2021 and 2022, respectively, compared with 2017. Additionally, biochar application combined with N fertilizer remarkably enhanced the decrease in BD compared with biochar application alone in all 6 years of the study.

Fig. 1
figure 1

Effects of biochar one-off application and different nitrogen level on soil (A) bulk density (BD), (B) total porosity (Tp), and (C) saturated hydraulic conductivity (Ks) in 2017, 2021 and 2022. T0, T1.5, T3.0 and T4.5 indicate biochar levels of 0%, 1.5%, 3.0%, and 4.5% w/w. N0 and N225 indicate no nitrogen applied and conventional nitrogen applied level. For each year and at each N treatment, different letters indicate significantly different between biochar application rates (p < 0.05). Error bars standard error of the mean

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The Tp value was ranked as T0 < T1.5 < T3.0 < T4.5 with or without N fertilization, and significant differences were observed among all biochar treatments in both 2021 and 2022 (Fig. 1B). Compared with 2017, Tp increased by 12.03% in 2021 and 14.84% in 2022, indicating an increasing trend of biochar effect with application time. Biochar application combined with N fertilizer led to higher Tp compared with the treatment of biochar application alone in all experimental years. Moreover, the interaction between biochar treatment and nitrogen level significantly influenced Tp.

The Ks was affected by biochar treatment, nitrogen level, year, and their interaction (Fig. 1C). In all experimental years, biochar application, either alone or in combination with N fertilizer, significantly and consistently increased Ks value compared with no biochar treatment. The annual average of Ks values in T1.5, T3.0, and T4.5 treatments increased by 3217.78%, 4405.57%, and 5539.83%, respectively, compared with T0. A significant difference was observed among all biochar treatments in 2017, 2021 and 2022. The average Ks value was 10.76% and 32.92% higher in 2021 and 2022, respectively, compared with 2017. Furthermore, the average Ks value in biochar application combined with N fertilizer treatment increased by 3.63% to 8.52% compared with biochar application alone.

3.1.2 Soil aggregates

Biochar application significantly enhanced the formation of macro-aggregates (> 2 mm, and 0.25–2 mm) compared with T0 but decreased the formation of micro-aggregates (0.053–0.025 mm, and < 0.053 mm) with or without N fertilization in all experimental years. No significant differences were found among T1.5, T3.0, and T4.5 in 2017, nor between T3.0 and T4.5 in 2021 and 2022. The average macro-aggregate was 25.15% and 28.60% higher in 2021 and 2022 than in 2017, whereas the micro-aggregate decreased by 9.14% and 11.38%, respectively. The synergistic application of biochar and N fertilizer significantly improved soil aggregate distribution compared with biochar application alone. Additionally, the interaction between biochar, nitrogen, and year significantly affected the distribution of soil aggregates.

The MWD and PWSA are two important indexes to evaluate the stability of soil aggregates. The MWD significantly increased with the increase in biochar application rate with or without N fertilization in all experimental years. All biochar amendment treatments differed significantly from the no biochar treatment (T0) (Fig. 2B). The average PWSA values in T1.5, T3.0, and T4.5 increased by 17.79%, 27.17%, and 40.47%, respectively, compared with T0, with noticeable differences across all biochar treatments (Fig. 2C). The average MWD and PWSA were 10.87% and 15.10% higher in 2021, and 15.22% and 18.23% higher in 2022 than in 2017, respectively, indicating a significant increasing trend of the effectiveness of biochar with application time. Additionally, the combined application of biochar and nitrogen fertilizer exhibited significantly greater impacts on both MWD and PWSA, compared with the application of biochar alone. The interaction between biochar treatment and nitrogen level had substantial effects on MWD (Fig. 2B), whereas the interaction between biochar, nitrogen, and year significantly affected PWSA (Fig. 2C).

Fig. 2
figure 2

Effects of biochar one-off application and different nitrogen level on soil (A) aggregate size distribution, B) mean weight diameter (MWD), (C) percentage of water stable aggregates (PWSA), and (D) principal component analysis (PCA) in 2017, 2021 and 2022. T0, T1.5, T3.0 and T4.5 indicate biochar levels of 0%, 1.5%, 3.0%, and 4.5% w/w. N0 and N225 indicate no nitrogen applied and conventional nitrogen applied level. For each year and at each N treatment, different letters indicate significantly different between biochar application rates (p < 0.05). Error bars indicate standard error of the mean

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The results of principal component analysis (PCA) indicated that PC1 and PC2 contributed to 97.9% and 1.0% of the total variation based on soil composition, respectively (Fig. 2D). The PCA findings emphasized the considerable impact of biochar application on the composition of aggregates in highly saline-alkali soil. The effect of T4.5 on the aggregate component of soil was relatively similar to that of T3.0 treatment and had a higher score in PC1.

3.2 Soil salt ion concentration and chemical properties

3.2.1 Na+, K+, Ca2+, and Mg2+ concentrations, Na+/K+ ratio, and Na+/Ca2+ ratio

The concentration of Na+ in the soil significantly decreased with the increase in biochar application rate in all 6 years of the study (Fig. 3A). The average Na+ concentration in T1.5, T3.0, and T4.5 reduced by 21.37%, 28.99%, and 37.20%, respectively, compared with T0. A significant difference was observed in Na+ concentration among T0, T1.5, T3.0, and T4.5 treatments in all 6 years of this study. The soil Na+ concentration showed a decreasing trend with the increase in the years of biochar application. The average Na+ concentration was 22.06% and 25.23% lower in 2021 and 2022, respectively, compared with 2017. Additionally, the combination of biochar application and nitrogen fertilizer resulted in an 11.27% lower Na+ concentration than biochar application alone. The average soil K+ concentration in T1.5, T3.0, and T4.5 treatments increased by 172.14%, 219.41%, and 301.28%, respectively. A noticeable difference was observed among all the biochar treatments (Fig. 3B). However, the soil K+ concentration did not increase with the increase in the years of biochar application; instead, it showed a decreasing trend from 2017 to 2022. The values in 2021 and 2022 decreased by 18.72% and 21.74%, respectively, compared with those in 2017. Additionally, biochar combined with N fertilizer significantly elevated the K+ concentration compared with the biochar application alone. The soil Ca2+ and Mg2+ concentrations increased with the increase in biochar application rate in all 6 years of the study, with or without N fertilization, and significant differences were observed among all biochar treatments (Fig. 3C, D). However, soil Ca2+ and Mg2+ concentrations decreased significantly with the extension of biochar application years. In 2021 and 2022, the soil Ca2+ concentration decreased by 12.36–14.12%, and the Mg2+ concentration decreased by 5.09–6.77%, respectively, compared with 2017. The combined use of biochar and nitrogen fertilizer had significantly greater impacts on soil Ca2+ and Mg2+ concentrations than biochar application alone. The results of three-way ANOVA showed that the concentrations of soil Na+, K+, and Ca2+ were significantly affected by biochar treatment, nitrogen level, year, and their interaction. However, Mg2+ concentration was affected by only biochar treatment, nitrogen level, and year.

Fig. 3
figure 3

Effects of biochar one-off application and different nitrogen level on soil Na+ (A), K+ (B), Ca2+ (C), Mg2+ (D), Na+/K+ ratio (E), and Na+/Ca2+ ratio (F) in 2017, 2021 and 2022. T0, T1.5, T3.0 and T4.5 indicate biochar levels of 0%, 1.5%, 3.0%, and 4.5% w/w. N0 and N225 indicate no nitrogen applied and conventional nitrogen applied level. For each year and at each N treatment, different letters indicate significantly different between biochar application rates (p < 0.05). Error bars indicate standard error of the mean

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The study examined the soil Na+/K+ (Fig. 3E) and Na+/Ca2+ ratios (Fig. 3F), revealing a consistent ranking across treatments: T0 < T1.5 < T3.0 < T4.5, irrespective of N fertilization. Significant differences were found among T0, T1.5, T3.0, and T4.5. Over the years, a decreasing trend in soil Na+/K+ and Na+/Ca2+ ratios was observed, although no significant difference was noted between 2021 and 2022. Notably, the combined application of biochar and nitrogen fertilizer was more effective in reducing the Na+/K+ and Na+/Ca2+ ratios. The results of three-way ANOVA underscored the substantial influence of the interaction between biochar treatment and year, as well as the interaction between biochar treatment and nitrogen level, on the Na+/K+ and Na+/Ca2+ratios.

3.2.2 Soil pH, ECe, ESP, SARe, and CEC

The effect of biochar application treatment on soil pH was not consistent across different years (Fig. 4A). In 2017, an increasing trend of soil pH was observed following biochar application, although no significant difference was noted among different biochar application treatments. However, as the duration of biochar application increased, a significant decrease in soil pH was observed, compared with T0. Notably, no significant difference was observed among T1.5, T3.0 and T4.5. A consistent downward trend of pH was evident with the increase in the years of biochar application. In 2021 and 2022, soil pH decreased by 4.15% and 4.43%, respectively, compared with 2017. The pH reduction effect was significantly enhanced when biochar was combined with N fertilizer compared with biochar application alone. Additionally, significant interaction effects on soil pH were observed among biochar, nitrogen fertilizer, and year.

Fig. 4
figure 4

Effects of biochar one-off application and different nitrogen level on soil (A) pH, (B) electrical conductivity of saturated paste extract (ECe), (C) exchange sodium percentage (ESP), (D) sodium absorption ratio of saturated paste extract (SARe), and (E) cation exchange capacity (CEC) in 2017, 2021 and 2022. T0, T1.5, T3.0 and T4.5 indicate biochar levels of 0%, 1.5%, 3.0%, and 4.5% w/w. N0 and N225 indicate no nitrogen applied and conventional nitrogen applied level. For each year and at each N treatment, different letters indicate significantly different between biochar application rates (p < 0.05). Error bars indicate standard error of the mean

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The soil ECe and ESP significantly decreased after biochar application, with or without N fertilization, in all 6 years of the study and decreased significantly with an increase in biochar application (Fig. 4B, C). Compared with T0, the average ECe and ESP in T1.5, T3.0, and T4.5 decreased by 48.91–71.01% and 49.72–68.02%, respectively. Significant differences were found among all treatments in all experimental years. Soil EC and ESP decreased significantly with the increase in biochar application years. The average ECe and ESP were 19.50% and 18.25% lower in 2021, and 27.32% and 24.49% lower in 2022 than in 2017, respectively. The treatment of biochar combined with nitrogen fertilizer was more effective in decreasing ECe and ESP. The ECe and ESP for biochar combined with nitrogen fertilizer were 16.66% and 10.93% lower, respectively, than those for biochar application alone. Three-way ANOVA showed that the interaction between biochar, nitrogen and year significantly affected the ECe and ESP (Fig. 4B, C). The soil SARe was obviously affected by biochar treatment, nitrogen level, year, and interaction between biochar treatment and year (Fig. 4D). Compared with T0, the average SARe in T1.5, T3.0, and T4.5 was reduced by 50.90%, 55.26%, and 61.67%, respectively. The soil SARe was 19.50% and 27.32% lower in 2021 and 2022, respectively, than in 2017. The combination of biochar and nitrogen fertilizer was more beneficial in reducing the soil SARe.

In contrast to soil pH, ECe, ESP, and SARe, soil CEC exhibited a significant increase with the rise in biochar application rate, with or without N fertilization, throughout all 6 years of the study (Fig. 4E). The differences among all biochar application treatments reached a significant level. The soil CEC value also increased with the duration of biochar application. The soil CEC was 9.67% and 11.56% higher in 2021 and 2022, respectively, than in 2017. In addition, the combined treatment of biochar and nitrogen fertilizer resulted in a 5.37% higher CEC compared with biochar application alone. Three-way ANOVA indicated that biochar treatment, nitrogen level, year, and their interaction significantly affected the soil CEC.

3.3 Soil nutrient properties

3.3.1 Soil TN, AN, AP, AK, and SOM

The biochar, nitrogen level, year, and their interaction significantly affected the soil TN and AN (Table 2). The TN increased significantly with the increase in the biochar application rate. Compared with T0, TN increased by 31.16% under T1.5, 56.77% under T3.0, and 81.29% under T4.5, with significant differences observed among all biochar application treatments in all experimental years. As the years of biochar application increased, soil TN showed a significant upward trend, with an increase of 19.92% in 2021 and 24.73% in 2022 compared with 2017. Additionally, the combined application of biochar and nitrogen fertilizer enhanced the soil TN content. However, the soil AN content was inconsistent across different years of biochar application. With or without N fertilization, the soil AN content was ranked as T0 > T1.5 > T3.0 > T4.5 in 2017, while it was ranked as T0 < T1.5 < T3.0 < T4.5 in 2021 and 2022. The soil AN content was 60.49% and 62.73% higher in 2021 and 2022, respectively, compared with 2017, indicating a significant increasing trend with the duration of application time. Moreover, the combined application of biochar and nitrogen fertilizer was more beneficial in increasing the AN content.

Table 2 Effects of biochar one-off application and different nitrogen level on soil nutrient properties in 2017, 2021 and 2022
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The contents of AP and AK exhibited a consistent increase with the increase in biochar application rates across all 6 years of the study (Table 2). Compared with T0, the application of biochar at different rates (T1.5, T3.0, and T4.5) led to a substantial increase in the AP and AK contents, ranging from 52.23% to 194.20% for AP and 87.14% to 309.58% for AK. These increases demonstrated significant differences among all treatments. Moreover, the contents of AP and AK showed an increasing trend with the duration of biochar application. The combined application of biochar and nitrogen fertilizer also increased the AP and AK contents. This combined treatment resulted in a 21.84% increase in soil AP content and a 10.54% increase in AK content compared with biochar application alone.

The SOM exhibited a distinct ranking across biochar application treatments, with or without N fertilization: T4.5 > T3.0 > T1.5 > T0. A significant difference was observed among all biochar treatments (Table 2). The SOM exhibited an increasing trend with the increase in the biochar application years. The soil SOM increased by 9.52% and 10.88% in 2021 and 2022 respectively, compared with 2017. In addition, applying biochar combined with nitrogen fertilizer resulted in a 10.74% higher SOM than the biochar application alone. The results of the three-way ANOVA underscored the SOM significant influence of biochar, nitrogen, year, and the interaction between biochar treatment and nitrogen level on SOM level in the soil.

3.3.2 Soil enzyme activity

The ANOVA revealed that catalase (Fig. 5A), phosphatase (Fig. 5B), urease (Fig. 5C), and sucrase activities (Fig. 5D) were obviously affected by biochar, nitrogen level, year, and their interaction. The catalase activity was in the order T4.5 > T3.0 > T1.5 > T0 with or without N fertilization. A significant difference was observed among all biochar treatments. The soil catalase activity was 17.93% and 20.43% higher in 2021 and 2022, respectively, than in 2017. The combined application of biochar and nitrogen fertilizer more effectively improved soil catalase activity; the catalase activity of biochar combined with nitrogen fertilizer was 37.22% higher than that of biochar application alone. Biochar application significantly increased the activity of soil phosphatase. Compared with T0, the average phosphatase activity in T1.5, T3.0, and T4.5 increased by 20.04%, 30.60%, and 45.04%, respectively. Biochar-amended treatments (T1.5, T3.0, and T4.5) exhibited significantly higher phosphatase activity than those without biochar (T0) in all 6 years of the study. The combined application of biochar and nitrogen fertilizer had a more significant effect on improving soil phosphatase activity. The phosphatase activity was 26.38% higher in biochar combined with nitrogen fertilizer treatment compared with biochar application alone. Urease activity gradually increased with the increase in the biochar application rate in the order T4.5 > T3.0 > T1.5 > T0, with or without N fertilization. A significant difference was observed between biochar application treatments (T1.5, T3.0, and T4.5) and no biochar treatment (T0). Soil urease activity showed a significant upward trend with the increase in biochar application years, rising by 31.90% and 35.21% in 2021 and 2022, respectively, compared with 2017. The urease activity of biochar combined with nitrogen fertilizer was 27.28% higher than that of biochar application alone. The sucrase activity in T1.5, T3.0, and T4.5 increased by 24.34%, 45.59%, and 52.31%, respectively. Compared with 2017, soil sucrase activity increased by 12.01% and 14.34% in 2021 and 2022, respectively. Biochar, in combined with nitrogen fertilizer, was more beneficial in increasing sucrase activity.

Fig. 5
figure 5

Effects of biochar one-off application and different nitrogen level on soil catalase activity (A), phosphatase activity (B), urease activity (C), and sucrase activity (D) in 2017, 2021 and 2022. T0, T1.5, T3.0 and T4.5 indicate biochar levels of 0%, 1.5%, 3.0%, and 4.5% w/w. N0 and N225 indicate no nitrogen applied and conventional nitrogen applied level. For each year and at each N treatment, different letters indicate significantly different between biochar application rates (p < 0.05). Error bars indicate standard error of the mean

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3.4 Biomass yield (BY), grain yield (GY), and harvest index (HI)

The biochar application significantly increased BY, which further increased with the increase in biochar application rates (Table 3). The BY in T1.5, T3.0, and T4.5 increased by 74.50%, 90.89%, and 102.91%, respectively, compared with T0. BY showed an annually increasing trend with the increase in biochar application years. The BY was 17.24% and 19.07% higher in 2021 and 2022, respectively, than in 2017. The combined application of biochar and nitrogen fertilizer was more effective in increasing BY; the BY of biochar combined with nitrogen fertilizer was 134.06% higher than that of biochar applied alone. The three-way ANOVA showed that the BY was significantly influenced by biochar, nitrogen level, year, interaction of biochar and year, and the interaction of biochar and nitrogen (Table 3).

Table 3 Effects of biochar one-off application and different nitrogen level on rice biomass yield (BY), grain yield (GY) and harvest index (HI) in 2017, 2021 and 2022
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The GY increased by 116.38% under T1.5, 141.24% under T3.0, and 145.20% under T4.5 compared with T0 (Table 3). In the absence of N fertilization, an obvious difference was found among all biochar treatments; however, under N fertilization treatment, no obvious difference was observed among T1.5, T3.0, and T4.5 treatments in all 6 years of the study. The GY exhibited a consistent increasing trend with the duration of biochar application. The average GY was 11.39% and 14.37% higher in 2021 and 2022, respectively, compared with 2017. Combing biochar with nitrogen fertilizer was more effective on increasing the GY, with a 451.38% increase compared with biochar application alone. Additionally, the interaction between biochar, nitrogen and year significantly affected GY.

Compared with T0, the average HI increased by 33.32% under T1.5, 46.01% under T3.0, and 42.04% under T4.5 (Table 3). The HI was significantly influenced by biochar application, nitrogen level, and their interaction, but it was not obviously affected by year. In addition, biochar, in combination with nitrogen fertilizer, was more beneficial to increasing the harvest index.

3.5 Correlation among yields, soil physicochemical properties, salt ion concentrations, nutrient properties, and enzyme activities

Figure 6 shows the relationships among yields, soil physicochemical properties, salt ion concentrations, nutrient properties, and enzyme activities. The correlation analysis results revealed that BY and GY were positively correlated with Ks, Tp, MWD, CEC, SOM, TN, AN, AP, AK, and soil enzyme activities. Conversely, they exhibited negative correlations with BD, pH, ESP, ECe, SARe, Na+, Na+/K+ ratio, and Na+/Ca2+ ratio. Furthermore, BD showed negative correlations with CEC, MWD, K+, Ca2+, Mg2+, SOM, TN, AN, AP, AK, and soil enzyme activities, but positive correlations with the pH, ECe, ESP, SARe, Na+, Na+/K+ ratio, and Na+/Ca2+ ratio. The Tp, Ks, and MWD demonstrated negative correlations with the Na+, ECe, ESP, SARe, Na+/K+ ratio, and Na+/Ca2+ ratio, but positive correlations with CEC, soil nutrients, and enzyme activities. In addition, soil enzyme activities positively correlated with soil nutrients, Ks, Tp, MWD, and CEC. However, they exhibited negative correlations with BD, pH, ECe, ESP, SARe, Na+, Na+/K+ ratio, and Na+/Ca2+ ratio. SOM demonstrated positive correlations with soil Tp, Ks, macro-aggregates, MWD, CEC, K+, Ca2+, Mg2+, TN, AP, and AK, but negative correlations with BD, micro-aggregates, ESP, ECe, SARe, and Na+ concentration.

Fig. 6
figure 6

Pearson correlation analysis among yield, soil physicochemical parameters, salt ion concentration, nutrients and enzyme activity. BD bulk density, TP total porosity, Ks saturated hydraulic conductivity, MWD mean weight diameter, ESP soil exchangeable sodium percentage, ECe electrical conductivity of soil saturation extract, SARe sodium adsorption ratio of soil saturation extract, CEC cation exchange capacity, TN total nitrogen, AP available phosphorus, AK available potassium, SOM soil organic matter, BY biomass yield, GY grain yield, HI harvest index

Full size image

4 Discussion

4.1 Long-term effects of the one-off biochar application on the physical properties of soil

Excessive Na+ concentrations can induce the slaking, swelling, and dispersion of saline-alkali soil (Lakhdar et al. 2009). The condition leads to the destruction of soil aggregates, decrease in soil permeability and hydraulic conductivity, and the development of poor soil structure (Oster et al. 1999). Previous studies have demonstrated that biochar addition could significantly improve the physical properties of saline soil by balancing water content and porosity, increasing SOC and CEC, and stabilizing soil aggregate structure (Gunarathne et al. 2020; Wang et al. 2023b). This study identified a similar trend. A one-off application of biochar to highly saline-alkali paddy soils led to a significant reduction in soil BD during a 6-year study period. The magnitude of the decrease in BD was directly linked to the rate of biochar application (Fig. 1A). Additionally, the addition of biochar obviously enhanced the Tp and Ks of highly saline-alkali paddy soils across all 6 years of the study (Fig. 1B, C), and a one-off higher rate of biochar application demonstrated a more pronounced positive effect on improving Tp and Ks. Over the years of biochar application, the improvement in soil BD, Tp, and Ks became more significant (Fig. 1). These findings demonstrated that one-off application of biochar had a significant positive impact on the physical characteristics of highly saline-alkali paddy soils, particularly in case of high biochar application rates. Improvements in these physical characteristics facilitated the leaching of Na+ from soil layers (Saifullah et al. 2018) and fostered root growth (Li et al. 2022b). Moreover, the remedial effect of biochar was intensified with each successive year of application. Several mechanisms contributed to these results: (i) Biochar, characterized by its high specific surface area and porous structure, ameliorated the soil BD, Tp, and Ks of salt-affected soil through both a dilution effect and intricate interactions with soil particles (Huang et al. 2019; Sun et al. 2021; Wang et al. 2023a). (ii) The annual increase in SOM resulting from biochar addition (Table 2) and a large amount of root residue (Li et al. 2022b) could potentially enhance the development of soil aggregate structure (Fig. 2) and enhance microbial activity in the rhizosphere (Kolton et al. 2016). (iii) The considerable presence of Ca2+ in biochar resulted in a significant increase in soil Ca2+ concentration upon application (Fig. 3C), which was beneficial to the formation of soil aggregates. (iv) The slow degradation characteristics and good structural stability of biochar in soils made it particularly suitable for the long-term improvement and maintenance of soil structure (Chaganti and Crohn 2015; Wang et al. 2023a). Similarly, previous studies reported that biochar improved the soil Tp, and Ks, and increased crop yield over a 2-year trial under salinity stress caused by saline water intrusion (Huang et al. 2019; Phuong et al. 2020).

Soil aggregate stability serves as a key indicator for assessing the construction, degradation, and stabilization of soil structure (Liu et al. 2021). Saline-alkali soils often exhibit suboptimal soil aggregation due to insufficient organic matter input and the dispersion of soil clay particles caused by excessive Na+ (Lakhdar et al. 2009; Liu et al. 2017a). A review analysis suggested that biochar effectively increased soil aggregate stability in salt-affected lands (Saifullah et al. 2018). However, limited information is available regarding the long-term impacts of biochar addition on soil aggregate stability in highly saline-alkali paddy fields. In this study, a one-off application of biochar, especially in combination with nitrogen fertilizer, significantly heightened the ratio of macro-aggregates, MWD, and PWSA in highly saline-alkali paddy soils during the 6-year study period, particularly with higher biochar dosages (Fig. 2). Additionally, the positive impact of biochar on soil aggregates became more pronounced with an extended year of biochar application (Fig. 2C). This could be attributed to the following mechanisms: (i) the improvement in soil BD, Tp, and Ks occurred year by year (Fig. 1), which was beneficial to the leaching of sodium ions from the soil profile; (ii) the input of bivalent cations, particularly Ca2+ (Fig. 3C) and Mg2+ (Fig. 3D), contributed to the dissociation of Na+ from the soil exchange complex (Fig. 3A) and facilitated the creation of macro-aggregates, especially with longer term application (Santos et al. 2021; Fig. 2C); (iii) the continuous input of SOM from biochar application(Table 2) and root residue (Li et al. 2022b), particularly under the treatment of biochar combined with nitrogen fertilizer, benefited soil nutrient transformation and assisting in the combination of multivalent cations and clay particles (Gu et al. 2022); and (iv) biochar acting as a favorable habitat for soil microorganisms (Saifullah et al. 2018), significantly rhizosphere microbial activity and function, including bacteria, mycorrhizal fungi, and actinomycetes. This led to a large amount of mycelium and root secretions, promoting the formation of soil macro-aggregates by binding clay minerals and organic matter particles (Bronick and Lal 2005; Wang et al. 2023b). The correlation analysis further revealed positive relationships of macro-aggregates and MWD with Tp, Ks, SOM, Ca2+, Mg2+, CEC, soil nutrients, and soil enzyme activities (Fig. 6). Hence, it could be concluded that a one-off application of biochar, especially when applied in conjunction with nitrogen fertilizer, in highly saline-alkali paddy soils not only improved soil aggregate stability but also sustained this beneficial effect in the long term. These results aligned with previous studies indicating that biochar application improved soil aggregate stability in salt-affected drylands (Chaganti and Crohn 2015; Chen et al. 2020; Huang et al. 2019). Gong et al. (2021) also observed an interaction between biochar and nutrients in soda saline-alkali grassland. These findings might be beneficial to enhancing rice production in highly saline-alkali fields in the future.

4.2 Long-term effects of the one-off application of biochar on soil salt concentrations and chemical properties

Removing excessive sodium ions from the cation exchange sites of colloids and subsequently leaching them out of the root zone soils constitute the primary approach to alleviating saline-alkali stress and rehabilitating saline-alkali soils (Oster et al. 1999; Rengasamy 2010). Previous studies indicated that adding biochar could effectively reduce the Na+ concentration in salt-affected soils through exchange adsorption and by increasing the availability of divalent cations, such as Ca2+ and Mg2+ (Li et al. 2022a; Phuong et al. 2020; Wang et al. 2023a). However, most studies focused on short-term field trials (1–3 years) using continuous application patterns. The results of this study, indicated that a one-off application of biochar significantly reduced Na+ concentration (Fig. 3A), Na+/K+ ratio (Fig. 3E), and Na+/Ca2+ ratio (Fig. 3F), while enhancing Ca2+ (Fig. 3C) and Mg2+ (Fig. 3D) concentrations during the 6-year period. Moreover, the increasing adverse effects of biochar on Na+ concentration with the increasing years of application suggested that a one-off application of biochar could continuously reduce Na+ content, effectively improving the physicochemical properties of soil (Figs. 1, 2 and 3). The reduction in Na+ concentration within highly saline-alkali paddy soils might be attributed to several factors induced by biochar: (i) acceleration of the leaching rate of Na+ from the soil profile through enhancing the physical properties of soil (Fig. 3), (ii) binding of Na+ to biochar due to its strong sorption capacity (Cui et al. 2021), (iii) release of bivalent cations, such as Ca2+ and Mg2+, facilitating the removal of Na+ from cation exchange sites and soil solution, followed by leaching out from the topsoil (Saifullah et al. 2018; Fig. 3A), and (iv) the increase in SOM, TN, AP, and AK in all 6 years of the study, and AN in 2021 and 2022 (Table 2) resulting in improved organic acid secretion by roots, further promoting the release of Ca2+ and Mg2+ into the soils (Li et al. 2022b). Moreover, biochar helps improve the binding of small particles in highly saline-sodic paddy soils to form large water-stable aggregates (Fig. 2), thereby increasing soil Tp (Fig. 1B), enhancing the soil Ks (Fig. 1C) and facilitating the leaching of more Na+ from the soil profile (Lakhdar et al. 2009). In conclusion, biochar could effectively ameliorate the physicochemical traits of highly saline-alkali soils, offsetting and mitigating the stress caused by salinity and alkalinity. Similar findings were reported by Huang et al. (2019) in a 3-year investigation of sweet corn cultivation in saline-sodic upland soils. However, contrary to these findings, the present study revealed that the concentrations of Ca2+ and Mg2+ did not exhibit an increasing trend with the application year of biochar. Specifically, the concentrations of Ca2+ and Mg2+ in 2017 were elevated compared with those in 2021 and 2022. This might be attributed to the initial rapid release of Ca2+ and Mg2+ from the applied biochar into the soil in the first year of application (Nan et al. 2022). Moreover, the combined application of biochar and nitrogen fertilizer exhibited a more pronounced impact on the concentrations of Na+, K+, Ca2+, and Mg2+ ions compared with biochar application alone. This could be attributed to the synergistic effects of applying both biochar and nitrogen fertilizer. These effects include: (i) providing a large amount of nitrogen sources for microbial activities (Sun et al. 2022; Table 2), improving the soil C/N ratio (Yao et al. 2021), which could significantly enhancing soil enzyme activity (Fig. 5), improving soil physical and chemical structure (Figs. 1, 2, 3 and 4), promoting the mineralization and decomposition of organic matter (Xu et al. 2023), and reducing the fixation and loss of Ca2+, Mg2+, and K+ (Saifullah et al. 2018); and (ii) meeting the nitrogen needs for rice root growth, enhancing the biomass of root (Chen et al. 2021), and stimulating the secretion of organic acids and other compounds (Li et al. 2022b; Zhou et al. 2021), which is beneficial to the re-release of fixed ions in the saline-alkali soil environments and boosting the amount of organic matter returned to the field (Table 2), ultimately increasing the source and bioavailability of Ca2+, Mg2+, and K+ in the saline-alkali paddy soils (Fig. 3). Previous studies also indicated that the combined application of biochar and nitrogen fertilizer was more conducive to stimulating soil microbial activities (Sun et al. 2022), facilitating nutrient conversion (Gong et al. 2021; Gu et al. 2022; Xu et al. 2023), and promoting root growth (Chen et al. 2021).

Numerous studies demonstrated that the pH of salt-affected soils significantly increased after applying of alkaline biochar (Liu and Zhang 2012; Gunarathne et al. 2020). On the contrary, other studies showed that the applying of biochar could reduce the pH of salt-affected soils by reducing the soil ESP (Lashari et al. 2015; Zhao et al. 2020; Zhou et al. 2021). The present 6-year study demonstrated that the pH value of highly saline-alkali paddy soils treated with biochar exhibited an increasing trend in 2017 despite a marked decrease in ESP. Similar results were obtained by Sun et al. (2016), who reported that the soil pH was unchanged after amendment with maize stalk biochar (pH 8.0) and peanut shell biochar (pH 7.7) in saline soils (pH 8.6) in a pot experiment. This could be attributed to the larger, one-off application of biochar in 2017 (the field application rates of biochar were 20.25, 40.50, and 60.75 t ha−1). However, as the years of biochar application increased, a notable reduction in soil pH was observed under biochar treatments compared with no biochar treatment (Fig. 4A). Similar results were reported by Liu and Zhang (2012) in an incubation experiment. Several factors might contribute to these results: (i) the continuous enhancement of soil Tp (Fig. 1B), Ks (Fig. 1C), and CEC (Fig. 4E) annually after one-off application of biochar (Fig. 1), which was beneficial to Na+ leaching from the soil profile; (ii) the rapid annual reduction of Na+ (Fig. 3A) after biochar application resulting in a significant decrease in ESP (Fig. 4C), SARe (Fig. 4D) and ECe (Fig. 4B); (iii) biochar application facilitating the dissociation of H+ from the ion exchange complex through the release of K+, Ca2+, and Mg2+ (Santos et al. 2021); and (iv) the continuous annual improvement in soil physicochemical traits (Figs. 1, 2, 3) following biochar application enhancing the production of organic acids, such as malic acid and glutamic acid, as well as root exudates, particularly in the presence of nitrogen fertilizer (Li et al. 2022b; Zhou et al. 2021). Moreover, our previous study also showed that biochar significantly decreased the concentrations of CO32− and HCO3− in saline-alkali soil (Li et al. 2022a). The initial pH of biochar may play a significant role in biochar-induced changes in the pH of salt-affected soils (Saifullah et al. 2018). The pH value of the peanut shell biochar used in this study was 7.94 (Table 2), significantly lower than the soil pH (10.10), which might be another contributing factor. Saifullah et al. (2018) revealed that the effects of biochar on the pH of salt-stressed soil depended on the type of biochar, the amount applied, and the type of salt-affected soil. Additionally, the combination of biochar and nitrogen fertilizer was more beneficial to reducing the pH of highly saline-alkali paddy soils in 2021 and 2022. This was because it was more favorable for improving soil physical (Fig. 1) and chemical properties (Figs. 3 and 4), soil aggregate structure (Fig. 2), nutrient availability, and SOM (Table 2).

Saline-alkali stress can also affect soil chemical properties such as EC, ESP, SAR, and CEC. The initial values of ECe, ESP, SARe, and CEC for the soils in this study were 24.08 dS m−1, 71.11%, 368.11 (mmolc L−1)1/2, and 10.99 cmol kg−1, respectively (Table 1), indicating severe inhibition of crop growth. Previous studies consistently highlighted the positive effects of biochar in decreasing ESP, EC, and SAR, while concurrently enhancing CEC in salt-affected soils (Gunarathne et al. 2020; Li et al. 2022a; Wang et al. 2023b; Zhang et al. 2020). However, it is crucial to acknowledge that a majority of these studies were conducted as short-term field trials (1–3 years) or laboratory incubation experiments, with limited data on the long-term impacts of one-off biochar application on ECe, ESP, SARe, and CEC in highly saline-alkali paddy soils. In this study, a one-off biochar application significantly lowered ECe (Fig. 4B), ESP (Fig. 4C), and SARe (Fig. 4D), while increasing CEC (Fig. 4E) in highly saline-alkali paddy soils throughout the study period. Moreover, the positive effects of biochar on ECe, ESP, SARe, and CEC were enhanced with increasing years of biochar application, particularly when combined with nitrogen fertilizer. Therefore, it was concluded that a one-off application of biochar, particularly at high rates (T3.0 and T4.5), was an effective approach to mitigating salt accumulation in the topsoil, alleviating saline-alkali stress, and enhancing nutrient retention in highly saline-alkali paddy soils. One-off application of biochar improved saline-alkali paddy soils in the long term. Several mechanisms contributed to this positive impact: (i) Biochar directly provided large amounts of exogenous Ca2+ (Fig. 3C), Mg2+ (Fig. 3D), and K+ (Fig. 3B) while also indirectly promoting the release of divalent cations from soil colloids through ion exchange, acid–base regulation, decomposition of organic matter, and microbial activities, consistently and effectively replacing Na+ from soil colloids (Saifullah et al. 2018; Zhou et al. 2021). (ii) The soil’s physical traits (Figs. 1 and 2), particularly soil aggregates, Tp and Ks, steadily improved annually after the one-off application of biochar, facilitating the Na+ leaching from the soil profile (Fig. 3A). (iii) Biochar could absorb substantial amounts of Na+ in the soil layer through ion exchange, chemical complexation, and surface adsorption, facilitated by the negatively charged carboxylic and phenolic functional groups present on its particle surface (Palansooriya et al. 2019). (iv) Biochar obviously enhanced soil nutrients annually (Table 2), facilitating the leaching of Na+ from the exchange sites of soil colloids and promoting root growth (Cui et al. 2021; Li et al. 2022b). The structural stability and slow mineralization efficiency of biochar in soils might account for the long-term beneficial impact on the chemical properties of soil (Nan et al. 2022; Saifullah et al. 2018). The correlation analysis further revealed a negative correlation between ESP, ECe, and SARe with Tp, Ks, macro-aggregates, MWD, soil nutrients, SOM, CEC, and enzyme activities (Fig. 6).

4.3 Long-term effects of the one-off application of biochar on soil nutrient properties and enzyme activity

Saline-alkali soils typically suffer from limited nitrogen availability and utilization due to (i) compromised physical, chemical, and biological properties of soil (Morsy et al. 2022; Qadir et al. 1997), (ii) insufficient input of organic matter (Kamran et al. 2021), and (iii) substantial nitrogen volatilization loss to the atmosphere in high salinity environments (Qadir et al. 1997; Wang et al. 2023a). Recent studies demonstrated that incorporating biochar into salt-affected soil significantly improved the soil’s nitrogen bioavailability (Gunarathne et al. 2020; Liu et al. 2022; Zhao et al. 2020). This improvement occured directly through nitrogen release and indirectly by enhancing the abundance and activity of microbial populations (Gong et al. 2021; Rasse et al. 2022; Saifullah et al. 2018). In this study, a significant increase in TN was observed in highly saline-alkali paddy soils after a one-off application of biochar. Moreover, the positive effect of biochar on TN became more pronounced with each subsequent year of application (Table 2). This result underscored the potential of a one-off biochar application in improving nitrogen retention in highly saline-alkali paddy soils, particularly at high application rates (T3.0 and T4.5). This positive effect demonstrated persistence and stability over time. The increase in TN could be attributed to the rapid nutrient release of nutrients from biochar (Agbna et al. 2017; Chen et al. 2020; Yao et al. 2021). Another contributing factor could be the enhancement of microbial activity and community sizes in soils due to biochar application, leading to increased nitrogen immobilization and reduced nitrogen leaching (Saifullah et al. 2018; Wang et al. 2023b). Unlike the consistent increase in TN observed across all 6 years of the study, AN showed a different trend. Specifically, the AN significantly decreased in the first year of biochar application (in 2017). However, the AN showed a significant increase (in 2021 and 2022) with the subsequent increase in the years of biochar application. Possible reasons for this trend include (i) the rapid increase in microbial activity after biochar application, thus promoting the uptake of soil microorganisms for AN (Chen et al. 2020; Saifullah et al. 2018; Wang et al. 2023b); (ii) the improvement of soil physiochemical properties (Figs. 1, 2 and 3) significantly promoting the root growth (Li et al. 2022b; Zhou et al. 2021) and rice biomass (Table 3), thus increasing the amount of nitrogen absorbed by root system in the first year; and (iii) more effective release of the nutrients contained in biochar in the second year (Agbna et al. 2017; Chen et al. 2020; Yao et al. 2021). Besides, biochar significantly promoted soil nitrogen cycling, and reduced nitrogen loss from salt-affected soils annually (Bhaduri et al. 2016; Wang et al. 2023a). Agbna et al. (2017) reported similar findings, observing a notable reduction in AN within the initial year of wheat straw biochar incorporation.

The availability of P and K in salt-affected soils is significantly inhibited by the high salinity and alkaline conditions, along with the degradation of soil structure, reduced SOM, and decreased soil microbial activity (Qadir et al. 1997; Saifullah et al. 2018). Recent studies suggested that the short-term addition of biochar (1–3 years) could significantly increase the availability of P and K in salt-affected fields, both directly supplying these nutrients to the soils and indirectly by ameliorating the physicochemical and biological characteristics of soil (Mehdizadeh et al. 2020; Wang et al. 2023b). In this study, a significant increase in the concentrations of AP and AK was observed in highly saline-alkali paddy soils during the 6-year study period, especially with high rates of biochar application (T3.0 and T4.5). The positive effect on AP and AK continued to increase with the increase in the years of biochar addition (Table 3). Moreover, combining biochar with nitrogen fertilizer had a more pronounced impact on increasing soil AP content than biochar application alone. This was mainly because biochar served as a source of soil P and K (the P and K contents in biochar used in this study were 0.74 and 12.53 mg g−1, respectively). Besides, biochar addition, especially combined with N fertilizer treatments, significantly improved the physicochemical properties of soil (Figs. 1, 2 and 3), such as Tp, Ks, macro-aggregates, pH, ESP, SARe, and CEC (Fig. 4). It also enhanced the SOM (Table 3) and enzyme activity (Fig. 5) in highly saline-alkali paddy soils. Furthermore, biochar-induced generation of dissolved organic carbon (DOC) and humic substances can block the sorption sites on clay particles, minimizing the formation of calcium phosphate crystal phases, and then reducing phosphorus fertilizer fixation and loss (Taghavimehr 2015; Wang et al. 2023b). Liu et al. (2017b) observed that the relative abundance and distribution of phosphate-solubilizing bacteria such as Thiobacillus, Pseudomonas, and Flavobacterium significantly increased after biochar application, which might be another reason for the enhancement of P availability in salt-affected soils. The correlation analysis revealed a negative correlation of AP and AK with soil BD, pH, ESP, SARe, and ECe, while exhibiting a positive correlation with Tp, Ks, macro-aggregates, MWD, SOM, CEC, and enzyme activities (Fig. 6). Hence, it was inferred that the one-off application of biochar in highly saline-alkali paddy soils led to continuous improvement in soil AP and AK contents, as well as a decrease in the Na+/K+ and Na+/Ca2+ ratios (Fig. 3). Furthermore, this positive effect exhibited remarkable stability and persistence (Table 3). However, Xu et al. (2016) reported that the precipitation of phosphorus in soil was enhanced by biochar, resulting in an increase in soil pH and subsequently restricting the availability of phosphorus in salt-affected soil. In this study, soil pH showed a slight increasing trend (although the difference was not significant), while the soil AP exhibited a significant upward trend following the one-off application of biochar in 2017 (Fig. 4A). This could be attributed to factors such as (i) the initially low background levels of AP (9.13 mg g−1) and SOM (0.64%), and high pH (10.10) in the highly saline-alkali paddy soils used in this study, (ii) the rapid release of a substantial amount of phosphorus carried by the biochar itself after its one-off application in 2017, (iii) the significant reduction in phosphorus fertilizer retention and loss due to the adsorption effects of biochar through exchanges (Wang et al. 2023b), and (iv) the improvement of the root zone environment promoting the activity of soil phosphatase (Fig. 5B). However, as the years of biochar application increased, soil pH showed a significant decline (Fig. 4A). This might be another reason for the increase in P bioavailability in 2021 and 2022. Saifullah et al. (2018) and Wang et al. (2023a) also reported that the positive and adverse effects of biochar on the P bioavailability might be related to biochar feedstock, pyrolysis temperature and duration, and soil types.

SOM plays a crucial role in supplying nutrients to plants and improving the physicochemical characteristics of soil. However, in saline-sodic soil, the SOM content tends to be relatively low due to its poor physicochemical and biological properties, high pH, and elevated salt levels (Kamran et al. 2021; Wang et al. 2009). Previous studies have indicated that the incorporating biochar can improve SOC, and maintain the SOM balance (Bohara et al. 2018; Gong et al. 2021; Rasse et al. 2022). However, long-term studies investigating the impact of one-off biochar application on SOM in highly saline-alkali paddy soils are scarce. In this study, biochar application significantly increased the SOM content in highly saline-alkali paddy soils, particularly at high biochar addition rates. The positive impact of biochar on SOM increased with the number of years of biochar addition (Table 2). Furthermore, the combined application of biochar with nitrogen fertilizer had a significantly greater impact on SOM than the application alone (Table 2). This can be attributed to two factors. First, biochar has a high affinity for adsorbing soil organic molecules, facilitating their catalytic activity and promoting the polymerization of small organic molecules into SOM (Palansooriya et al. 2019). Second, the slow aging process of biochar contributes to the formation of humus, which enhances soil fertility over time (Saifullah et al. 2018). A comparable result was obtained by Glaser et al. (2001), who observed that SOM in soil with biochar in tropical areas was more resistant to decomposition under high-temperature and high-humidity conditions than other forms of organic matter due to its high chemical and biological stability. In addition, the increasing return of organic materials, such as roots, to the field after biochar application is also an essential factor (Li et al. 2022b; Wang et al. 2023a). The correlation analysis revealed that SOM exhibited positive correlations with soil Tp, Ks, macro-aggregates, MWD, CEC, N, AP, AK, and soil enzyme activities. In contrast, negative correlations were observed with BD, pH, ESP, ECe, and SARe (Fig. 6). Therefore, it was concluded that one-off biochar application had a continuous and significant positive impact in terms of increasing SOM, contributing to the improvement in soil physical properties (Fig. 1), enhancement of soil macro-aggregates (Fig. 2), amelioration of soil chemical characteristics (Fig. 4), and provision of a C source for soil enzyme (Fig. 5) and microbial activities (Cui et al. 2021), thereby relieving the saline-alkali stress.

Soil enzyme activity is closely related to the physicochemical characteristics of soil, including pH, SOM, and nutrient cycling (Song et al. 2012). In addition, the substantial deposition of Na+ in salt-affected soils has a negative impact on soil enzyme activity (Shi et al. 2019). Therefore, it is crucial to thoroughly understand the changes in soil enzyme activity for the reclamation and reutilization of saline-alkali lands. However, studies on the long-term effect of biochar one-off application on enzyme activity in highly saline-alkali paddy soils are limited. The data in this study revealed a significant enhancement in the activities of catalase (Fig. 5A), phosphatase (Fig. 5B), urease (Fig. 5C), and sucrase (Fig. 5D) after the one-off biochar application, with an increasing positive influence of biochar on soil enzyme activity over the years of biochar addition. These enhancements may contribute to the continuous acceleration of SOM formation and the intensification of nutrient recycling and transformation (Cui et al. 2021; Gunarathne et al. 2020). These improvements are primarily attributed to the enhancements in the physicochemical properties of soil (Figs. 1, 2, 3 and 4), SOM, soil nutrient content (Table 2), and microorganism diversity and richness (Saifullah et al. 2018). The correlation analysis revealed that soil enzyme activity exhibited positive associations with soil Tp, Ks, macro-aggregates, CEC, SOM, and nutrient availability while showing negative correlations with soil BD, Na+ content, pH, ESP, ECe, and SARe (Fig. 6). Furthermore, the soil enzyme activity was significantly higher when biochar was combined with N fertilizer compared with biochar application alone (Fig. 5). This indicated that the synergistic effect of biochar combined with N fertilizer on soil enzyme activity was significant. Similar findings were reported in saline soils by Liu et al. (2017a).

4.4 Long-term effects of the one-off biochar application on rice yield

The primary factors contributing to reduced crop yield under saline-alkali stress are nutrient deficiencies, ionic toxicity, and osmotic stress (Chi et al. 2012; Morsy et al. 2022). Multiple studies have documented that biochar application can effectively enhance crop yield by improving soil physicochemical parameters, enhancing nutrient availability, promoting soil microbial activity, and optimizing root morphological and physiological traits (Cui et al. 2021; Joseph et al. 2021; Wang et al. 2023b; Zhou et al. 2021). Previous studies have shown that the long-term continuous application of biochar can significantly improve soil physicochemical properties and increase crop yield (Gu et al. 2022; Liu et al. 2020; Sun et al. 2021). However, the long-term effects of the one-off biochar application on rice yield in heavy saline-alkali soil have not been reported. This study demonstrated that the one-off biochar application into highly saline-alkali paddy fields significantly increased BY, GY, and HI during the 6-year study period, especially at higher biochar application rates (T3.0 and T4.5). Moreover, the positive influence of biochar on BY and GY increased with the increase in the number of years of biochar addition (Table 3). The underlying mechanisms could be as follows: (i) one-off application of biochar persistently improved soil BD (Fig. 1A), Tp (Fig. 1B), Ks (Fig. 1C), and soil macro-aggregates (Fig. 2), and increased the divalent cation input, especially Ca2+ and Mg2+ (Fig. 3), which could significantly promote the removal of Na+ from the soil exchange complex (Fig. 3A); (ii) one-off application of biochar significantly alleviated saline-alkali stress by decreasing ECe (Fig. 4B), ESP (Fig. 4C), and SARe (Fig. 4D); (iii) the soil pH significantly decreased in 2021 and 2022 (Fig. 4A); (iv) the soil nutrient bioavailability and SOM were continuously enhanced after the one-off application of biochar (Table 2), improving the physiochemical properties of soil (Figs. 1, 2 ,3 and 4), enhancing the C-N cycle (Rasse et al. 2022; Saifullah et al. 2018), and strengthening soil enzyme activity (Fig. 5); and (v) the HI significantly increased after biochar addition (Table 3), helping transport photosynthetic C source into rice grains (Li et al. 2022a). On the other hand, our previous studies indicated that biochar effectively lowered the Na+ concentration in rice leaves, enhanced leaf K+ concentration (Jin et al. 2018), improved leaf water status (Piao et al. 2022; Ran et al. 2019), and optimized root structure and function (Li et al. 2022b), thus obviously promoting leaf photosynthetic rate and net assimilation rate (Piao et al. 2022). The correlation analysis revealed that GY exhibited positive correlations with soil Ks, Tp, macro-aggregates, MWD, CEC, SOM, soil nutrients, and soil enzyme activities, and negative correlations with BD, pH, ESP, ECe, SARe, Na+, Na+/K+ ratio, and Na+/Ca2+ ratio.

In addition, GY increased by 451.38% under biochar and nitrogen fertilizer combination treatment compared with biochar alone (Table 3). Various studies also reported similar findings, indicating increased rice yield when biochar was combined with nitrogen fertilizer in normal paddy soils (Liu et al. 2020; Sun et al. 2022; Gu et al. 2022). This synergistic effect could be attributed to the combined application of biochar and nitrogen fertilizer, which proves to be more effective than applying biochar alone in terms of providing a large number of nitrogen sources for the growth of soil microbial population and the rice plants (Table 2). These factors are more conducive to the growth of soil microorganisms (Sun et al. 2022; Zhang et al. 2024), enhance of soil enzyme activity (Fig. 5), increase of root exudates, optimization of rice root structure (Li et al. 2022b; Zhou et al. 2021), and the increase of SOM returning to the field (Table 2). Therefore, these positive changes further (i) improve the physical and chemical properties of soil (Figs. 1, 2 ,3 and 4), thereby facilitating the growth and absorption of root system (Chen et al. 2021); and (ii) increase the transformation and availability of soil nutrients, which can provide more nutrient supply for the growth and development of rice (Table 2; Fig. 3). Furthermore, combining biochar with nitrogen fertilizer proved beneficial in mitigating the harmful impact of saline-alkali stress on rice growth and development by regulating osmotic stress, increasing stomatal conductance, mitigating of ion toxicity, improving photosynthetic processes (Javed et al. 2021; Piao et al. 2022; Ran et al. 2019), and triggering N assimilation enzyme activities (the related results have not yet been published). Previous studies have also indicated that excessive N fertilizer application can inhibit crop growth due to nitrogen toxicity (Javed et al. 2021; Saifullah et al. 2018). Therefore, further investigations are warranted into the synergistic effect and dose–effect relationship between biochar and nitrogen fertilizer. The GY was in the order T3.0 > T4.5 > T1.5 > T0 under N fertilizer treatment, and no significant difference was found among T3.0, T4.5, and T1.5. The increase in yield with 4.5% (w/w) biochar amendment was lower than that with 3.0% (w/w) primarily due to the immobilization of AN (Table 2) and a lower HI (Table 3). Therefore, the results unequivocally demonstrated the potential of the one-off application of biochar to effectively and consistently enhance rice productivity in highly saline-alkali paddy soils, with the application rate of 3.0% (w/w) identified as the most effective.

5 Conclusions

In this 6-year field study, the one-off application of biochar effectively improved the physical properties of soil, leading to continuous reductions in soil BD, and increased Tp, Ks, soil macro-aggregates, MWD, and PWAS. Simultaneously, the one-off application of biochar continuously proved beneficial in reducing soil Na+ concentration, Na+/K+ ratio, Na+/Ca2+ ratio, ECe, ESP, and SARe, while significantly increasing soil CEC annually. It played a key role in alleviating saline-alkali stress. Furthermore, the one-off application of biochar consistently enhanced the fertility and enzyme activities of highly saline-alkali paddy soils, except for the AN in the first year. This indicates that the one-off application of biochar had a positive and sustainable effect in terms of improving soil function and primary productivity in highly saline-alkali paddy soils. The one-off application of biochar, especially when combined with nitrogen fertilizer, resulted in a significant increase in rice yield throughout the 6-year study period. The positive impact of biochar on rice yield showed an upward trend with each successive year of biochar application, highlighting its potential for long-term and sustainable improvement in highly saline-alkali paddy soils. Considering the potential of biochar to improve soil quality and increase rice yield, the application rate of 3.0% (w/w) biochar emerged as the most effective rate. These findings provide valuable insights into the remediation and enhancement of soil health and agricultural productivity in highly saline-alkali paddy soils in Northeast China.