1 Introduction

Saline soils, sodic soils and saline-sodic soils are collectively called salt-affected soils (Singh et al. 2019) and adversely affect the growth of most crop plants. The estimated total area of salt-affected soils is approximately 1 billion hectares worldwide (Abbas et al. 2013; FAO-ITPS 2015). In the future, this number will continuously increase because of poor management of soil and water resources and regional and global climate change and variability (Ivushkin et al. 2019). It is expected that more than 50% of arable lands will encounter a serious salinity challenge by 2050 (Meng et al. 2016). To meet the challenges of global food security, the reclamation of saline croplands and the prevention of soil salinization have been urgently needed in global agricultural production (Biswas & Biswas 2014). Proper and cost-effective measures for decreasing the effects of salt stress on crop growth are thus a high-priority technological development need in global agriculture. Recent studies have highlighted biochar as a promising tool for improving crop productivity in salt-affected soils (Amini et al. 2016; Chaganti et al. 2015; Feng et al. 2017; Kim et al. 2016; Lashari et al. 2015).

Biochar, a carbon-rich material, has attracted considerable attention as an organic soil amendment to improve soil properties (physical, chemical and biological) by modulating soil conditions because of its large surface area, rich pore structure and high cation exchange capacity (Ding et al. 2017; Purakayastha et al. 2019). Similar to normal soils, many studies have indicated that biochar can strongly mitigate or even eliminate the stress effects of salt on plant growth in salt-affected soils (Elshaikh et al. 2018). However, Luo et al. (2017) reported that a relatively high rate of biochar application may produce negative results (e.g., inhibition of plant growth). The response of plant growth to biochar application is closely related to the properties of biochar (e.g., EC and pH of biochar) in salt-affected soils (Sun et al. 2016). The properties (e.g., salinity levels, pH and salt types) of salt-affected soils generally also impact the response of crop productivity to biochar application, which is considered by farmers when deciding whether to use biochar. The results of these previous studies showed that the current results on the reclamation of salt-affected soils with biochar addition are variable or inconsistent, which is probably because of the wide variety of biochar and soil properties analysed in these studies (Amini et al. 2016; Saifullah et al. 2018).

Additionally, evidence has shown that the benefits of biochar applied to restored soils depend on planting and management practices and the combination of biochar and climate factors (Jeffery et al. 2011; Smider and Singh 2014; Thomas et al. 2013). For example, the combination of biochar with different organic and inorganic amendments also results in controversial effects; further studies are needed to determine the best combinations suitable for salt-affected soils (Hammer et al. 2015; Saifullah et al. 2018; Xu et al. 2016). Several previous meta-analyses on the effects of biochar application on crop productivity and soil properties have been carried out (Jeffery et al. 2011; Biederman and Harpole 2013; Dai et al. 2020), but these analyses did not specify the salt-affected soils.

In salt-affected soils, a quantitative review should focus not only on the effects of biochar application on crop productivity but also on soil salinity. Soil salinity is essential for optimizing biochar production to improve crop productivity in salt-affected soils. Many studies have shown that biochar applied to salt-affected soils can decrease soil salinity (Chaganti et al. 2015; Sadegh-Zadeh et al. 2018; Thomas et al. 2013; Yue et al. 2016), which is explained by the improvement in soil hydrophysical properties (Edeh et al. 2020; Wong et al. 2010). However, an increase in soil salinity caused by biochar application was observed in salt-affected soils (Esfandbod et al. 2017), which may be mainly due to soluble salts in biochar (Yu et al. 2013). Therefore, elucidating the effects of biochar properties, climatic and soil conditions, and planting and management practices on crop productivity and soil salinity is critical for choosing suitable biochar or developing engineered biochar with specific functionality for salt-affected soils to promote crop productivity.

Meta-analysis is a quantitative statistical model that systematically analyses complex factors that affect the dependent variable of concern. Because these variables are obtained from published articles, meta-analysis increases the objectivity of systematic reviews from separate empirical research (Stanley 2001). Thus, in this study, we performed an MA of published literature data to quantify the effect of biochar with different characteristics on crop productivity and soil salinity. A comprehensive evaluation of published data is vital to provide a clear picture of biochar properties to help enhance its ability to improve crop productivity and soil salinity in salt-affected soils. The meta-analysis conducted in our study considered different studies involving a range of biochar properties, initial soil properties, climatic conditions, and planting and management practices. The results of the present study are essential for informing biochar applications and for proper science-based policy-making in salt-affected soils

2 Materials and methods

2.1 Data collection

Literature before June 2022 was collected and screened for meta-analysis in accordance with the following requirements: (1) the study must be original experimental research with quantitative results on the effects of biochar application on crop productivity or on the salinity of the soil where the crops are located; (2) basic information on soil properties (0~20 cm), experimental design and biochar application rate should be clearly reported in the studies; and (3) the same experiment must include an experimental group and a control group in which the experimental group was treated with biochar and the control group was not treated with biochar. The independent variables are consistent, and the number of experimental replications must be greater than or equal to 3. (4) When sampling results appeared multiple times in one study, the last sampling time was selected because this time is more representative of crop productivity and soil salinity.

Other basic data relevant to the biochar properties included the biochar electrical conductivity (EC), application rate, C/N ratio, pyrolysis temperature and pH, and the surrounding conditions included the mean annual temperature (MAT), mean annual precipitation (MAP), soil pH, soil salinity and types of salts. The rate of biochar application was standardized to t/ha, where values are given in % weight, and conversion was performed using the bulk density of the control treatments and depth provided. The planting practices, including cultivation, crop types and fertilizer types, were also extracted. Crop types were grouped as rice, wheat, maize and cash crops based on the main crop types in salt-affected agricultural land. For cultivation patterns, studies conducted in the laboratory were denoted as “pot” and those in the field as “field”. The fertilizer used in this study was classified into three groups: organic fertilizer, inorganic fertilizer and no fertilizer.

If the study presented the data in the form of text or a table, they were extracted directly. When the data were presented in the form of a graph, they were extracted with GetData Graph Digitizer 2.24. If the data provided in the study were the standard error (SE), the standard deviation (SD) was obtained by the equation \(SD=SE\sqrt{n}\), where "n" is the number of replications (Ren et al. 2019). If the standard deviation was missing from the data, it was calculated as 1/10 of the mean (Luo et al. 2006). All data were standardized: the biochar application rate unit was standardized to t·ha-1, the unit of soil salt content was standardized to g/kg, and the unit of crop productivity was standardized to kg·ha-1. For soil pH, if it was measured using the CaCl2 solution method, then \(pH({H}_{2}O)=1.65+0.86pH({CaCl}_{2})\) was used for conversion (Augusto et al. 2008). In total, 135 valid studies and 259 independent experiments were obtained, and the experimental areas are shown in Fig. 1.

Fig. 1
figure 1

Geographical distribution of the study areas

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2.2 Meta-analysis

Meta-analysis models were applied, in which the response ratio was modelled as a function of a study-level random effect and an explanatory factor variable (Wang et al. 2019). The advantage of using a response ratio is that it allows normalization of the experimental results on the same scale, which allows comparisons between different experiments. The disadvantage of this approach is that the interpretation of the results should be carried out with care because a wide range of covariables are not represented (Jeffery et al. 2015). In this study, a meta-analysis was used to study the effects of biochar on crop productivity and soil salinity. While crop productivity and soil salinity were affected by biochar, the crop growing environment and artificial disturbance also affected these factors. Therefore, to further study the combined effects of biochar properties, climatic and soil conditions, and planting and management practices on crop productivity and soil salinity, a submeta-analysis was designed to calculate the mean effect size of each biochar group under different index categories.

Meta-analysis was performed using Meta Win 2.1 software, with the response ratio (RR) as the statistic, and 95% confidence intervals (95% CIs) were calculated. The response ratio was calculated as \(-{x}_{e} /{-x}_{c}\), where \(-{x}_{e}\) and \({-x}_{c}\) are the mean values of the experimental group and the control group, respectively (Ren et al. 2018).

When RR > 1, the percentage is positive, indicating that the treatment has produced a positive effect. If RR < 1, the percentage is negative, indicating a negative effect; if RR=1, the percentage is 0, indicating that there is no difference between the experimental group and the control group. If the 95% CI is 0, the treatment group is not significantly different from the control group (Dai et al. 2020). To more clearly present the improvement effect of biochar, lnRR was used to describe the results of this study.

2.3 Data analysis

Microsoft Excel 2022 was used to establish the database and preprocess the data. Meta Win 2.1 was used for the meta-analysis. The integration analysis determined whether there was significant heterogeneity between treatments and between results (variability due to random error) (Ren et al. 2019). The effect size and the 95% CIs of each categorical group were calculated by Meta Win 2.1, and according to the results of the heterogeneity test, the chi-square test was used to test for data heterogeneity in this meta-analysis. When the test result exhibited p > 0.05, the homogeneity between different treatments or different research results was considered. The fixed effect model (FEM) was used to calculate the comprehensive effect value. Otherwise, the random effect model (REM) was used to calculate the comprehensive effect value. The smaller the standard deviation of RR is, the greater the weight assigned. Finally, the weighted response ratio (RR++) was obtained by weighting the RR of all experimental groups (Liu et al. 2014). According to the results of the meta-analysis, a forest plot was generated using GraphPad Prism 8 to visualize the results.

Linear regression analysis was performed using SPSS 16.0 to determine the correlations between crop productivity response and biochar properties/surrounding conditions and between soil salinity response and biochar/surrounding conditions. The results were plotted using Origin 2022.

3 Results

The results of the heterogeneity test (Table S2) demonstrated that significant heterogeneity existed in most of the studies included in the submeta-analysis because of the relevant data compiled from a wide range of studies under variable conditions, e.g., biochar production and experimental design.

3.1 Response of crop productivity and soil salinity to biochar properties

The electrical conductivity (EC), application rate, C/N ratio, pyrolysis temperature and pH of biochar all influenced the direction and magnitude of the crop productivity response in salt-affected soils. Biochar with an EC of 1–3 and > 5 mS/cm significantly increased crop productivity in salt-affected soils compared to soils without biochar (Fig. 2a). Biochar application rates of < 10, 10–20, 20–30, 30–40 and 40–50 t/ha were found to significantly increase crop productivity compared to the absence of biochar in salt-affected soils. However, a biochar application rate > 50 t ha-1 had no statistically significant effect on crop productivity in salt-affected soils (Fig. 2a). Biochar with a C/N ratio of 20–80 significantly increased crop productivity compared to the absence of biochar in salt-affected soils, whereas biochar with a C/N ratio of 0–20 and > 80 had no statistically significant effect on crop productivity. Biochar with a C/N ratio of 40–60 had the greatest increase in crop productivity in salt-affected soils (Fig. 2a).

Fig. 2
figure 2

Effect of biochar on crop productivity (a) and soil salinity (b) categorized by biochar property. Symbols indicate the mean percentage change of crop productivity relative to control with 95% confidence intervals. The y-axis shows the groups categorized by biochar properties correspondingly. The numbers after the name of group indicate the amount of pairwise comparison (with in brackets)

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Greater increases in crop productivity were observed with biochar produced at temperatures of 450–550 °C than with biochar produced at temperatures of < 450 °C or > 550 °C in salt-affected soils. Crop (plant) productivity responses (PPRs) were generally significantly positive (22% on average) with alkaline biochar ( > 7) or nonalkaline biochar ( < 7) in salt-affected soils. The greatest increase in crop productivity was recorded for biochar at pH 7–8, which was significantly greater than that for biochar at pH < 7 in salt-affected soils (Fig. 2a). The results of regression analysis demonstrated that the PPR was negatively correlated with the application rate of biochar. Other biochar properties showed nonsignificant correlations with the PPR (Fig. 3).

Fig. 3
figure 3

Regression analysis between biochar application and crop productivity response, (a) biochar application rate; (b) biochar C/N ratio; (c) biochar pyrolysis temperature; (d) biochar pH; (e) biochar electrical conductivity. The linear regression analysis was conducted using SPSS 16.0. The cases with p-value smaller than 0.05 indicate the significant relationship between biochar property and crop productivity response

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Biochar EC < 2 mS/cm significantly decreased soil salinity compared to that in the absence of biochar in salt-affected soils (Fig. 2b), while a biochar EC exceeding 2 mS/cm had no significant effect on soil salinity. A biochar application rate of < 30 t/ha had a negative effect on soil salinity (decreasing) in salt-affected soils, whereas a biochar application rate of > 30 t/ha had a positive effect on soil salinity (increasing) (Fig. 2b). Biochar application rates of < 10 and 20–30 t/ha significantly decreased soil salinity compared to the absence of biochar in salt-affected soils. Biochar with a C/N ratio > 80 significantly decreased soil salinity in salt-affected soils, whereas biochar with a C/N ratio < 80 had no statistically significant effect on soil salinity (Fig. 2b). The results of the regression analysis demonstrated that the response of soil salinity was negatively correlated (p = 0.013) with the C/N ratio of biochar but positively correlated with biochar pH. Biochar produced at pyrolysis temperatures < 450 °C significantly decreased soil salinity in salt-affected soils, whereas no significant effects were observed with biochar produced at pyrolysis temperatures > 450 °C. A biochar pH > 9 significantly increased soil salinity in salt-affected soils, while a biochar pH < 9 had no statistically significant effect on soil salinity (Fig. 2b).

3.2 Responses of crop productivity and soil salinity to climatic and soil conditions

Two climatic conditions (MAT and MAP) and three soil conditions (pH, salinity and type of salt) were selected to investigate the effects of biochar on crop productivity and soil salinity, and the results are illustrated in Fig. 5. With the exception of Cl-dominated salt-affected soils, climatic and soil conditions had significant positive effects on the response of crop productivity to biochar in salt-affected soils. There was no statistically significant difference in the effect of biochar application on salt-affected soils among the groups categorized by MAT. Greater increases in crop productivity were obtained for MAP < 400 mm in salt-affected soils, which was significantly greater than that for MAP > 800 mm (Fig. 5a).

In the present study, the application of biochar to salt-affected soils resulted in a statistically significant increase in crop productivity in all initial soil salinity, pH and salt type groups (except for chloride salt-affected soils) compared to the absence of biochar (Fig. 5a). The increase in crop productivity was significantly greater in soils with pH > 9 than in soils with pH < 7. The greatest increase in crop productivity was obtained in the soil with 3–5 g/kg salinity, but its variation was also greater. The increase in crop productivity was significantly greater in soils with > 10 g/kg salinity than in soils with < 3 g/kg salinity. The increase in crop productivity was also greater in sulfate-dominated salt-affected soils than in Cl-dominated salt-affected soils. However, no significant positive response was detected in the Cl-dominated salt-affected soils (Fig. 5b).

Increases in soil salinity were observed under conditions of MAT > 20 °C and MAP > 800 mm upon the addition of biochar to salt-affected soils (Fig. 5b). A significant increase in soil salinity was observed in soils with pH > 9 and < 7 upon the addition of biochar to salt-affected soils, while nonsignificant effects on soil salinity were observed in other soil pH groups (8–9 and 7–8). The effects of biochar on soil salinity were positive in soils with low salt content ( < 5), whereas negative effects of biochar on soil salinity were observed in soils with high salt content ( > 5). The greatest decreases in soil salinity caused by biochar application were observed in the sulfate-dominated salt-affected soils, which were significantly greater than those in the Cl-dominated salt-affected soils (Fig. 5b).

3.3 Responses of crop productivity and soil salinity to planting and management practices

Biochar application in both the pot and field trials had a statistically significant positive effect on crop productivity in salt-affected soils. Furthermore, the mean increase in crop productivity in the field trials was approximately two times greater than that in the pot trials. Significant increases in crop productivity were found to occur in cash crops, rice and corn upon the addition of biochar to salt-affected soils. Rice exhibited greater productivity in response to biochar application than did wheat and corn in salt-affected soils (Fig. 7a).

The “no fertilizer” group showed greater effects on crop productivity under the biochar treatment than under the treatment without biochar. The “organic fertilizer” and “inorganic fertilizer” groups exhibited greater effects on crop productivity when organic fertilizer or inorganic fertilizer was applied with biochar than when organic fertilizer or inorganic fertilizer were applied alone (control). Significant increases in crop productivity were observed when biochar was applied concurrently with inorganic fertilizer compared to when inorganic fertilizer was applied alone. There was no statistically significant effect of the concurrent application of biochar with organic fertilizer on crop productivity compared to that of organic fertilizer alone (Fig. 7a). Additionally, biochar applied concurrently with inorganic fertilizer led to significantly greater increases in crop productivity than biochar applied concurrently with organic fertilizer in salt-affected soils (Fig. 7a).

Significant increases in soil salinity were found to occur in cash crops and rice upon the addition of biochar to salt-affected soils (Fig. 7b). Biochar had negative effects on soil salinity in both wheat and corn, but the differences were not statistically significant. Compared with the application of organic fertilizer alone, significant decreases in soil salinity were observed when biochar was applied concurrently with organic fertilizer. However, there was no statistically significant effect of the concurrent application of biochar with inorganic fertilizer on soil salinity compared to that of inorganic fertilizer alone or when biochar was applied to soil without any fertilizer.

4 Discussion

4.1 Effects of biochar properties on crop productivity

In our meta-analysis of 259 independent experiments, we found that despite the variability in biochar properties, climate, soil properties, and production methods (or experimental conditions), the average effect of biochar application on crop productivity was significantly positive in salt-affected soils. The influence of biochar amendment on crop productivity depends, in part, on the specific qualities of the biochar, including the biochar EC, C/N, pH and pyrolysis conditions (Gang et al. 2012; Zhang et al. 2013). The present study showed that a significant increase in crop productivity was observed with an increase in the biochar application rate up to 50 t/ha in salt-affected soils, which may be due to a more effective amelioration of the adverse effects of salinity by a higher rate of biochar application (Singh et al. 2019). Consequently, the amount of available soil water increases, and osmotic stress is ameliorated (Akhtar et al. 2015; Haider et al. 2017). Biochar can increase the exchangeable K+ concentration and the K+:Na+ ratio in plants, which improves the salt tolerance of plants and subsequently improves plant growth (Lin et al. 2015). However, a negative effect on crop productivity was observed in salt-affected soils with biochar application rates > 50 t/ha. This indicated that excess application of biochar seemed to depress the growth of crops, which may be explained by lower N availability (Liu et al. 2017) and the increase in salinity (Singh et al. 2019b) at relatively higher biochar application. Baronti et al. (2010) found that a reduction in crop biomass was observed when the biochar application rate was > 60 t/ha. Some studies have reported that the excess amount of biochar applied to the soil may lead to an increase in soil salinity, especially in arid and semiarid regions, and inhibit plant growth (Baronti et al. 2010). The results of the regression analysis showed that crop productivity was negatively correlated (p < 0.01) with the biochar application rate (Fig. 3a). Nevertheless, considering the amended effect on crop productivity and economic cost in salt-affected soils, biochar application should be kept at an optimal level, such as an application rate < 50 t/ha.

When biochar with the lowest C/N ratio (0–20) was applied to salt-affected soils, there was no statistically significant increase in crop productivity compared to that in the absence of biochar. This was inconsistent with the study of Jeffery et al. (2017), who reported that biochar with the lowest C/N ratio (< 20) significantly increased plant productivity. Some studies have shown that N-rich biochar is not the source of N uptake (Gaskin et al. 2010; Rajkovich et al. 2012). The low C/N ratio of biochar (enriched in N content) contributed to the increase in the total soil N pool, but this N may be unavailable to plants (Biederman and Stanley Harpole 2013). Additionally, biochar with a low C/N ratio tended to have a high soil salinity in our study (Fig. 2b), which may have resulted in a decrease in crop productivity. Additionally, biochar with a C/N ratio exceeding 80 produced significantly lower crop productivity than biochar with a C/N ratio of 40–60 (Fig. 2a). Biochar with a high C/N ratio usually creates a temporary reservoir of organic N and consequently enhances the net immobilization of inorganic N (DeLuca et al. 2006), which might dampen the increase in crop productivity.

The liming effect of biochar applied to soils is generally considered to improve the availability of plant nutrients (e.g., P), which might contribute to an increase in crop productivity (Van Zwieten et al. 2009a). The present study revealed that biochar at pH < 7 or > 7 resulted in a statistically significant increase in crop productivity compared to the absence of biochar in salt-affected soils. This was inconsistent with a previous meta-analysis in which biochar with pH < 7 significantly decreased plant productivity (Dai et al. 2020; Liu et al. 2013), which was explained by the decrease in soil pH when biochar with pH < 7 was applied to acidic soils. In contrast, biochar with pH < 7 was applied to salt-affected soils with pH>7 and consequently neutralized the soil pH, which might have contributed to crop productivity. He et al. (2020) reported that the addition of acidic biochar to saline‒alkali soil significantly lowered the soil pH, thus increasing the availability of P for plants. The application of biochar produced at pyrolysis temperatures of 450–550 °C resulted in a greater increase in crop productivity than biochar produced at both < 450 °C and > 550 °C in our study (Fig. 2a). This may be due to the differences in the pH of the biochar among the three temperature groups. Low-temperature and high-temperature biochar have low and high pH values, respectively (Hossain et al. 2010). In the present study, biochar at both pH < 7 and > 9 resulted in a lower increase in crop productivity than biochar at pH 7–9 (Fig. 2a).

4.2 Effects of climatic and soil conditions on crop productivity

The results of previous studies suggested that climatic conditions also affected the response of crop productivity to biochar application (Atkinson et al. 2010; Ye et al. 2020). Our results showed that biochar applied to salt-affected soils resulted in significant increases in crop productivity in all the temperature and precipitation treatment groups. Nevertheless, a significantly lower increase in crop productivity was observed for MAP > 800 mm than for MAP < 400 mm in salt-affected soils with biochar application. This is supported by the negative correlation between the response of crop productivity and mean annual precipitation (Fig. 6b). This may be because biochar application resulted in a significantly greater increase in soil salinity at MAP > 800 mm than at MAP < 400 mm (Fig. 5b), which impacts crop growth. Additionally, plants are more sensitive to the improvement of soil water retention by biochar application in regions with low precipitation than in regions with high precipitation (Fischer et al. 2019; Edeh et al. 2020), which also contributes to a great increase in crop productivity. Biochar application resulted in a significantly greater soil water capacity (30%) in the region with MAP < 400 (Zhang et al. 2016) than in the region with MAP > 800 (3%) (Obia et al. 2016). In contrast, there was no statistically significant effect of biochar application on crop productivity between the temperature groups. This indicated that the annual average temperature is not a key factor restricting the effect of biochar application on crop productivity in salt-affected soils. This is supported by the nonsignificant correlation between the response of crop productivity and the mean annual temperature (Fig. 6a).

The initial salinity, pH and salt type of salt-affected soils are generally considered to influence the beneficial effects of biochar application. In the present study, compared with the absence of biochar, biochar application significantly increased crop productivity in salt-affected soils with initial pH < 7. This result was consistent with previous studies showing that biochar addition to soil resulted in an increase in soil pH through a liming effect (Van Zwieten et al. 2009b) and consequently promoted crop productivity (Dai et al. 2017). Moreover, a statistically significant increase in crop productivity caused by biochar application in salt-affected soils with initial pH > 8 (even pH > 9) was also observed in our study (Fig. 4a). This is because the pH values of the biochar used for saline soils are usually lower than the pH values of soil in many studies (Sun et al. 2016; He et al. 2020), which can also be observed in Fig. 2a (the number of biochar groups with pH < 7 is 106). Many previous studies have shown that the application of biochar at low pH might decrease the pH of salt-affected soils at high pH (Kim et al. 2016; Liu et al. 2017; Sun et al. 2016), which may contribute to the improvement of crop productivity. Dai et al. (2020) reported a positive response of crop productivity to biochar application in salt-affected soils.

Fig. 4
figure 4

Regression analysis between biochar property and soil salinity, (a) biochar application rate; (b) biochar C/N ratio; (c) biochar pyrolysis temperature; (d) biochar pH; (e) biochar electrical conductivity. The linear regression analysis was conducted using SPSS 16.0. The cases with p-value smaller than 0.05 indicate the significant relationship between biochar property and crop productivity response

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In the present study, biochar applied to salt-affected soils resulted in a significant increase in crop productivity in all initial soil salinity groups. This was consistent with a previous study that showed increased growth and yield of wheat plants with biochar amendment at each soil salinity level, which was explained by the mitigation of salinity stress (Akhtar et al. 2015). Additionally, salt-affected soils with an initial salinity > 10 experienced significantly greater increases in crop productivity via biochar application than did those with an initial salinity < 3 (Fig. 5a). This can be explained by the fact that biochar application has a negative effect on soil salinity (decrease in salinity) in salt-affected soils with high initial salinity (Fig. 5b), which benefits crop productivity in soils with high initial salinity. This mechanism was also confirmed by the positive correlation between the response of crop productivity and initial salinity in the present meta-analysis (Fig. 6d).

Fig. 5
figure 5

Effect of biochar on crop productivity (a) and soil salinity (b) categorized by climatic and soil conditions. Symbols indicate the mean percentage change of crop productivity relative to control with 95% confidence intervals. The y-axis shows the groups categorized by biochar properties correspondingly. The numbers after the name of group indicate the amount of pairwise comparison (with in brackets)

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Moreover, compared with the absence of biochar, the application of biochar to Cl-dominated salt-affected soils had no statistically significant effect on crop productivity, and biochar had a smaller effect on crop productivity than did the application of sulfate- or soda-dominated salt-affected soils. Sulfate-dominated salt-affected soils were significantly more permeable than Cl-dominated salt-affected soils (Sharma and Manchanda 1996) and consequently experienced increased water retention when irrigation or rainfall occurred, which improved crop productivity. Additionally, less exchangeable Na is found in sulfate-dominated salt-affected soils than in Cl-dominated salt-affected soils (Sharma and Manchanda 1996). These properties could result in lower soil salinity in sulfate-dominated saline soils than in Cl-dominated salt-affected soils. In the present study, a statistically significant decrease in soil salinity was observed in sulfate-dominated salt-affected soils with biochar application compared to that in Cl-dominated salt-affected soils, which could contribute to a greater increase in crop productivity. Conversely, the soda-dominated salt-affected soils with biochar application exhibited greater increases in crop productivity than did the Cl-dominated salt-affected soils. This may be due to two main reasons. First, soda-dominated salt-affected soils generally have a high soil pH, where biochar application could result in a greater increase in crop productivity compared to low soil pH (Fig. 5a). Second, a significant decrease in soil salinity was found in soda-dominated salt-affected soils with biochar compared to that in the same soils without biochar (Fig. 5b), which could also contribute to the increase in crop productivity.

4.3 Effects of planting and management practices on crop productivity

Our results show that rice had a greater productivity response to biochar application than did wheat and corn in salt-affected soils, which is inconsistent with the results obtained by Liu et al. (2013) and Ye et al. (2020). They reported a smaller response for rice crops than for crops of other types (i.e., maize and wheat). The differences may be related to soil water conditions, as most studies on rice were carried out under flooded conditions, in which the salt concentration of the soil solution is kept low and the pH of soils tends towards neutrality (Akinbile et al. 2016).

Significant increases in crop productivity were observed when biochar was applied concurrently with inorganic fertilizer compared to when inorganic fertilizer was applied alone. This finding is consistent with general recommendations in previous studies proposing the addition of inorganic fertilizer to further increase the positive effects of biochar application to soil (Yamato et al. 2006; Asai et al. 2009). In contrast, compared with the addition of organic fertilizer alone, the combined application of biochar and organic fertilizer had no statistically significant effect on crop productivity and had a significantly weaker effect on crop productivity than biochar applied concurrently with inorganic fertilizer. The high carbon applied when biochar is combined with organic fertilizer may cause net N immobilization, which also affects crop productivity (Bruun et al. 2012). Additionally, no significant difference was found in the effects of biochar on crop productivity between the “inorganic fertilizer” and “no fertilizer” groups (Fig. 7a). It seems likely that available N in salt-affected soils was not the limiting factor for crop productivity, as explained by previous additions of inorganic fertilizer (Jeffery et al. 2011).

4.4 Effects of biochar properties, climatic and soil conditions, and planting and management practices on soil salinity

In the present study, compared with the absence of biochar, biochar with EC < 2 mS/cm significantly decreased soil salinity in salt-affected soils (Fig. 2b). This may be because the response of soil salinity to the EC of biochar applied to salt-affected soils depends on the balance of salt input (the salt of biochar itself) and output (soil salt leaching). Low-salinity biochar results in greater leaching of salt than biochar alone, which decreases soil salinity in salt-affected soils (Chaganti et al. 2015; Saifullah et al. 2018). Biochar EC > 8 mS/cm had a positive effect on soil salinity compared to the absence of biochar. This may be because the high salt input from high-salinity biochar might offset soil salt leaching, which does not decrease soil salinity (Chaganti et al. 2015). This mechanism was also confirmed by the positive correlation between the response of crop productivity and initial salinity in the present meta-analysis (Fig. 4e).

A statistically significant decrease in soil salinity caused by biochar application rates of 20–30 and < 10 t/ha was observed in salt-affected soils compared to soils without biochar (Fig. 2b). This may be due to salt adsorption and salt leaching by biochar applied to salt-affected soils (Akhtar et al. 2015; Yue et al. 2016). Nevertheless, biochar at an application rate > 30 t/ha tended to have a positive effect on soil salinity in salt-affected soils (Fig. 2b), which may be due to the release of biochar ions, which offset the beneficial effect of biochar. Abrishamkesh et al. (2015) and Singh et al. (2019) reported a significant increase in salinity with an increasing rate of biochar application in salt-affected soils, explained by the presence of soluble salts from biochar decomposition (Yu et al. 2013) and the strong sorption of salt onto biochar followed by gradual desorption (Thomas et al. 2013).

Biochar with a C/N ratio > 80 significantly decreased soil salinity in salt-affected soils compared to that in soils without biochar. This may be because biochar with a high C/N ratio potentially has a high porosity and can absorb more salt and consequently decrease soil salinity. The results of the regression analysis demonstrated that the response of soil salinity was negatively correlated (p = 0.013) with increasing biochar C/N ratio (Fig. 4b). There was a statistically significant decrease in soil salinity in response to the application of biochar only at pyrolysis temperatures < 450 °C compared to that in the absence of biochar. This may be due to the high cation exchange capacity, which results in a high concentration of exchangeable Ca2+/Mg2+ replacing Na+ (Lashari et al. 2013) and thus decreasing soil salinity. Biochar pH > 9 significantly increased soil salinity in salt-affected soils. This result occurred because biochar with a high pH usually has a high salt content, which consequently increases soil salinity. This mechanism was also confirmed by the positive correlation (p = 0.022) between the response of crop productivity and initial salinity in the present meta-analysis (Fig. 4d).

One of the main concerns of salt-affected soil restoration is the change in soil salinity when biochar is applied to soils under different climatic and soil conditions. Biochar applied to salt-affected soils resulted in a statistically significant increase in soil salinity at high MATs ( > 20 °C) and MAPs ( > 800 mm). This is due to the rapid decomposition of biochar at relatively high temperatures and in moist environments (Nguyen and Lehmann 2009; Fang et al. 2014), which can increase soil salinity by releasing soluble biochar salts (Singh et al. 2019b). The results of the regression analysis showed that the response of soil salinity was positively correlated with increasing mean annual temperature (p < 0.01) and mean annual precipitation (p < 0.01) (Fig. 6b). Our study revealed that biochar applied to salt-affected soils had positive effects (increases of 5% and 13%) on soil salinity at initial salinities < 5 g/kg. This was inconsistent with many previous studies showing that a significant decrease in the soil salinity of salt-affected soils is caused by the application of biochar (Chaganti et al. 2015; Yue et al. 2016). They explained that salt adsorption and salt leaching by biochar applied to salt-affected soils reduce soil salinity. Akhtar et al. (2015) reported that the incorporation of biochar into salt-affected soils could alleviate salinity stress mainly because of its high salt adsorption potential, decreasing osmotic stress by enhancing the soil moisture content and releasing mineral nutrients. However, these studies have often been performed on salt-affected soils with high initial salinities. In contrast, Luo et al. (2017) reported that biochar added to soils with low initial salinity increases soil salinity because biochar contains a high ash content, which partly offsets the beneficial effects (reducing salinity) of biochar application. Sun et al. (2016) also reported that soil salinity increased when biochar was applied to soils with a low initial salinity. This mechanism was also confirmed by the negative correlation (decreasing trend in soil salinity) between the response of soil salinity and initial salinity in the present meta-analysis (Fig. 6h).

Fig. 6
figure 6

Regression analysis between surrounding conditions and crop productivity response, (a) mean annual temperature; (b)mean annual precipitation; (c) soil pH; (d) soil salinity (initial soil). Regression analysis between surrounding conditions and soil salinity response, (e) mean annual temperature; (f) mean annual precipitation; (g) soil pH; (h) soil salinity (initial soil). The linear regression analysis was conducted using SPSS 16.0. The cases with p-value smaller than 0.05 indicate the significant relationship between biochar property and crop productivity response

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Significant increases in soil salinity were found to occur in cash crops and rice upon the addition of biochar to salt-affected soils (Fig. 7b). This may be related to irrigation for cash crops and rice and consequently increased soil water moisture, which caused rapid biochar decomposition and increased soil salinity through the release of biochar ions. Compared with the addition of organic fertilizer alone, the combined application of biochar and organic fertilizer significantly decreased the soil salinity (Fig. 7b). This may be because the improvement in soil structure (porosity and aggregation) by the addition of organic fertilizer maximized the beneficial effects of biochar application on the decrease in soil salinity. Thus, the application of biochar combined with fertilizer (inorganic or organic) is recommended for restoring salt-affected soils. Taken together, these results indicate that biochar applied to salt-affected soils can improve crop productivity based on the analysis of the main soil properties.

Fig. 7
figure 7

Effect of biochar on crop productivity (a) and soil salinity (b) categorized by planting and management practices. Symbols indicate the mean percentage change of crop productivity relative to control with 95% confidence intervals. The y-axis shows the groups categorized by biochar properties correspondingly. The numbers after the name of group indicate the amount of pairwise comparison (with in brackets)

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5 Conclusion

The meta-analysis conducted in the present study showed that the overall effects of biochar on crop productivity and soil salinity were estimated to be 13% to 37% and -20% to 20%, respectively. However, the efficiency of biochar in improving crop productivity and soil salinity could be greatly affected by biochar properties, climatic and soil conditions, and planting and management practices. Furthermore, the underlying mechanisms contributing to the positive crop productivity response also changed for the same biochar under different climatic and soil conditions and plant and management practices. For example, the combined application of biochar with fertilizer (either organic or inorganic) did not cause a change in soil salinity and even significantly decreased soil salinity compared to the application of biochar alone in salt-affected soils. In sulfate-dominated saline soils with salinities exceeding 3 g/kg and soil pH values of 7–9, 40–50 t/ha of 40–60% C/N biochar is ideal for improving crop productivity and soil salinity under climatic conditions of MAT < 20 °C and MAP < 800 mm. The findings of the present study contribute to a better understanding of the effects of biochar on improving crop productivity and soil salinity in salt-affected soils. In summary, to enhance effective biochar utilization in salt-affected soils, biochar properties should be selected carefully before their application, considering the climatic and soil conditions and specific planting and management practices of the area.