1 Introduction

The rapid growth of the world population and loss of arable land pose a major threat to agricultural production and global food security (Dai et al. 2020; Farrar et al. 2018). Intensive farming and application of chemical fertilizers to the limited agricultural soils have led to adverse effects on the environment (Gao et al. 2022; Mukhina et al. 2020; Song et al. 2020), including deteriorating air quality due to the release of excessive amounts of aerosols and greenhouse gases, as well as contamination of soil and water bodies with excessive nutrients and heavy metals. Additionally, excessive fertilizer use can lead to soil degradation by depleting soil organic matter and decreasing soil fertility, which can result in soil texture deformation and decreased water holding capacity (Khorram et al. 2020). Recent studies have shown that approximately 2–10% of nutrients from chemical fertilizers (CF) travel from the soil to water bodies via surface runoff and subsurface leaching, which is a major contributor to eutrophication, and thereby reduces the use efficiency of fertilizers in plants (Ayele and Atlabachew 2021; Gao et al. 2022). Baligar and Fageria (2015) estimated that more than half of the applied nutrients (N, P, and K) were lost in the soil–plant system or were unavailable to plants. The negative effects of CF consequently can hinder plants’ growth and yield (Dai et al. 2020).

To promote sustainable soil management, it's crucial to maintain an adequate level of organic matter in the soil and ensure that the biological cycle of essential nutrients is not disrupted. Organic fertilizers, including composts, plant residue mulches, manures, and cover crops, can increase soil carbon and improve soil quality in accordance with sustainable agriculture and soil reinforcement (Khorram et al. 2020; Mukhina et al. 2020; Song et al. 2020). However, these organic amendments are often short-lived, and their decomposition rates are high, causing most of the organic matter added to mineralize into carbon dioxide (Bol et al. 2000). Furthermore, many organic amendments may act as a source of organic and inorganic pollutants, such as heavy metals, antibiotics, persistent organic pollutants, and microplastics to agricultural soil (Bolan et al. 2021; Sarkar et al. 2022). In response to these issues, researchers tried to use “biochar” that has similar positive effects on soil properties like organic manures or other organic amendments (Basak et al. 2022; Purakayastha et al. 2019; Woolf et al. 2010). Biochar is a carbon-rich, porous material made by pyrolyzing biomass feedstocks at various temperatures under low oxygen conditions (Dai et al. 2020; Khadem et al. 2021; Mukhina et al. 2020). Unlike soil organic matter, biochar does not decompose readily in soils due to its high resistance to degradation, which allows it to remain in the soil for centuries (Naeem et al. 2018).

Biochar has gained increasing attention as a soil conditioner due to its cost-effectiveness and environmentally friendly features, such as carbon sequestration and soil remediation (Gao et al. 2022; Khadem et al. 2021). It is widely accepted that the application of biochar can affect soil physical and chemical properties with a subsequent increase in plant growth and productivity (Gao et al. 2022; Hossain et al. 2021; Khadem et al. 2021). The above positive impacts of biochar occur mostly by increasing the availability and retention of both water and nutrients in the soil (Ayele and Atlabachew 2021; Hossain et al. 2021; Zhang et al. 2020), improving soil aggregation and porosity (Hossain et al. 2020; Videgain-Marco et al. 2020; Wali et al. 2020), enhancing plant nutrient uptake (Hossain et al. 2020; Solaiman et al. 2020; Song et al. 2020), and improving soil microbial activity (Hossain et al. 2021; Teutscherova et al. 2018). However, depending on specific soil attributes considered, the application of biochar to the soil may have no or slight negative effects on soil properties and conditions (Hossain et al. 2020; Karim et al. 2022; Zhang et al. 2020). For example, a meta-analysis showed that biochar application in soil reduced crop yields by 16% (Song et al. 2020). Nevertheless, due to its highly porous structure and surface functionalities, biochar can carry nutrients and supply them to plants as a slow-release fertilizer based on plant needs, resulting in decreased offsite loss of nutrients in the soil–plant system (Mandal et al. 2016; Wang et al. 2022).

Despite numerous reports on the effects of pristine biochars, few studies have investigated the impact of NEBC such as those enriched with nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) in the soil–plant system (Ahmad et al. 2018; Gunes et al. 2014; Karim et al. 2017, 2019, 2022; Wen et al. 2017; Zhang et al. 2017). Enrichment of biochar with multiple nutrients (N, P, and K) could be a desirable approach to improve the soil fertility, plant growth and yield. New biochar-based fertilizers are emerging and have demonstrated positive effects on crop yield and nutrient use efficiency (Buss et al. 2018; Gao et al. 2022; Hossain et al. 2021; Karim et al. 2022; Shi et al. 2020). However, there remains a lack of research on the effect of NEBC on soil properties, plant growth and development. Therefore, this study aims to bridge the existing gap by investigating the influence of NEBC on the physicochemical and biological properties of two contrasting soils, as well as the growth and development of canola plants (Brassica napus L.) in the biochar-amended soils. It was hypothesized that the addition of NEBC will positively influence soil physicochemical properties and plant growth, presenting a novel correlation between NEBC and soil–plant system enhancements.

2 Materials and Methods

2.1 Soil Collection and Initial Soil Properties

For this study, two types of surface soils (0–15 cm) were collected from Condobolin and Yenda in New South Wales, Australia. The soils were classified as grey Vertosol and red Kandosol according to the Australian Soil Classification system (Miltenyi et al. 2015), which are clay and clay loam types respectively. The initial properties of the soils are shown in Table 1. Both soils were low in nutrients such as available N, Olsen-P, available K, calcium (Ca), and magnesium (Mg). The grey Vertosol soil had a moderately alkaline pH of 8.13, while the red Kandosol was moderately acidic with a pH of 5.56. Both soils had low electrical conductivity and were non-saline. The collected soils were manually cleaned, air-dried, homogenized, and passed through a 2-mm sieve for the plant growth study.

Table 1 Selected properties of the soils used in the pot study
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2.2 Biochar Production and Characterization

Pristine biochars were produced from biosolids, cow manure, and chicken manure feedstocks. The feedstocks were air-dried, manually cleaned, ground, sieved to < 2 mm, and stored in airtight bags. Biochar was produced by slow pyrolysis at 300 ºC in a muffle furnace (Labec Muffle furnace, CEMLS-SD, Australia). Information regarding the chemical properties and nutrient contents of these biochars can be found in our previous publication (Hossain et al. 2021). NEBCs were synthesized by impregnating pristine biochars with urea fertilizer and KH2PO4. The NEBCs were obtained by heating the mixture of 10 g pristine biochar, 4 g urea fertilizer, and 8 g KH2PO4 in a sealed bottle with 100 mL Milli-Q water at 135 °C for 4 h, followed by air-drying for three days. The resulting NEBCs, derived from biosolids, cow manure, and chicken manure, were respectively labelled as nutrient-enriched biosolid biochar (NEBSBC), nutrient-enriched cow manure biochar (NECMBC) and nutrient-enriched chicken manure biochar (NEChMBC), respectively.

2.3 Experimental Design, Setup, and Sampling

A glasshouse pot experiment was conducted at the University of Newcastle, Callaghan campus, employing a completely randomized design. The experiment was designed to investigate the effects of three types of NEBCs (NEBSBC, NECMBC, and NEChMBC), one pristine biosolid biochar (BSBC), and CF comprising urea and KH2PO4), on two soil types (clay and clay loam) across three levels of biochar application (0%, 1%, and 5% w/w). The experiment was conducted in triplicates including the controls, and the pots were rotated at least once a week to account for changes in the glasshouse's environmental gradient. The plants were grown in individual plastic pots (11.8 cm height × 9 cm diameter), each filled with 0.5 kg of air-dried soil biochar and CF were added at three application levels: 0% (no addition), 1% w/w, and 5% w/w. Specifically, for the 1% w/w treatment, 5 g of either biochar or CF were thoroughly mixed with the soil. For the 5% w/w treatment, 25 g of either biochar or CF were similarly mixed with the soil. The soil was moistened with deionized (DI) water before sowing five canola seeds at a depth of 2 cm. After seven days, the germination percentage was calculated, and the plants were thinned down to four seedlings per pot. The experiment lasted eight weeks, during which the glasshouse conditions were maintained on a 16-h day and 8-h night cycle with temperatures of 25 and 18 °C during the day and night, respectively.

No seedlings emerged in both alkaline and acidic soils amended with 5% treatments, except for BSBC. Additionally, only 20% germination was observed in the clay loam soil (pH = 5.56) amended with 1% of all NEBCs, and the seedlings did not survive. With 1% CF amendment, no seeds germinated. Therefore, treatments that impeded or inhibited plant growth and seed germination were not included in the study's results, as there were likely toxic effects (Solaiman et al. 2012) of NEBC and CF. To investigate the inhibitory effect of seed germination, a separate germination trial was conducted using NEBSBC and CF treatments with five levels (0%, 0.05%, 0.10%, 0.25%, and 0.50%) and two replicates. Six seeds were sown per pot (5.5 cm height × 5 cm diameter), and the germination percentage was calculated on days 3, 5, 7, and 9 after sowing.

After six weeks of growth, the whole plants were harvested from the pots, and the shoots and roots were weighed for fresh above- and below-ground biomass yields. In addition, the shoot and root length of the plants were measured, and the above-ground plant materials were oven-dried, weighed for dry matter yield, and ground for nutrient analysis.

2.4 Soil and Plant Analyses

After harvesting the canola plants, the soil in each pot was collected, cleaned, air-dried, ground, and sieved through a 2-mm mesh for laboratory analysis. Basic characteristics of the soils were determined in a subsample following drying and homogenization. pH and electrical conductivity (EC) were determined by mixing 5 g of soil with 25 mL of Milli-Q water (1:5 ratio) and shaking for 1.5 h on a rotary end-over-end shaker (Rajkovich et al. 2011). After shaking, the samples were kept on the lab bench for 30 min before measuring the pH and EC values using a pH-EC meter. The soil moisture content was determined by the gravimetric method. For inorganic N analysis, air-dried soil samples were extracted with 2 M KCl solution at a 1:5 soil: solution ratio. For measuring the available inorganic N, specifically nitrate (NO3−) and ammonium (NH4+), ion chromatography was used. The Olsen-P was extracted by 0.5 M NaHCO3 (pH = 8.5) using a 1:20 dilution and shaken for 30 min on a rotary end-over-end shaker at room temperature (20 ± 2 °C). The available P was measured by the molybdenum blue method using a spectrophotometer at 712 nm. The exchangeable K+, Ca2+ and Mg2+ were extracted by 0.1 M BaCl2 using 1:60 dilution and two hours of shaking at room temperature (20 ± 2 °C). The major cations, including K, Ca, and Mg, were measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). Dehydrogenase activity (DHA) of soils was determined by the reduction of triphenyltetrazolium chloride to triphenylformazan, as described by Tan et al. (2017).

For measuring the elemental C, N and S concentrations in plant samples, a CNS elemental analyser was used following the dry combustion method (Bird et al. 2017). For determining the total P and K, the plant samples (mixed with 3 mL HNO3 and 2 mL H2O2) were digested in a microwave digester (MARS 6250/50, Matthews, NC, USA) and heated at 180 °C for 30 min at 1 kW power level. After cooling down the digestion unit at room temperature, the digested solution was used to measure total P and K using ICP-OES.

2.5 Statistical Analysis

Statistical analysis of the soil and plant data was conducted by a one-way analysis of variance (ANOVA) using SPSS (IBM SPSS Statistics version 27.0). Level of significance and post-hoc testing was performed through Tukey’s HSD method. The variability in the data was expressed as the standard deviation, and a value of p ≤ 0.05 was statistically significant at 95% confidence interval. The MS Excel 2016 was used for preparing the graphs.

3 Results

3.1 Effects of NEBC on Seed Germination

The effects of NEBSBC and CF on canola seed germination with clay soil and clay loam soils are presented in Table 2. The addition of NEBSBC showed inconsistent canola plants’ seed germination in both soils at the application rate of 0.0, 0.1, 0.25 and 0.5%. For clay soil, the addition of NEBSBC at different levels (0, 0.05, 0.1, 0.5%) did not affect seed germination, except 0.25% increased germination percentage over the CF treatment. For clay loam soil, application of 0.05, 0.1 and 0.25% NEBSBC showed lower seed germination percentages than CF treatment. However, the highest dose (0.5%) of NEBSBC had a higher (75%) germination percentage than the CF treatment. The germination percentage decreased by 20% at 0.05% and 0.5% addition, no effect was observed at 0.1% addition, and it increased by 33% at 0.25% addition of NEBSBC in the clay soil. The application of NEBSBC had no significant effect on canola seed germination percentage with the treatment that received no biochar(p < 0.05).

Table 2 Germination percentage of canola seeds at different days after sowing
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3.2 Effects of NEBC on Soil pH and EC

The effects of NEBSBC, NECMBC, NEChMBC, BSBC and CF on soil pH and EC are summarized in Table 3. In clay soils, the application of NEBC decreased the soil pH from 8.4 to 6.8, while in the CF-amended soil, the pH was reduced from 8.4 to 6.4. For clay loam soil, the soil pH was negatively correlated with the dose of BSBC, while the EC values of soil were significantly increased with the application of BSBC at 1 and 5% (p < 0.05). After harvesting the canola plants, the soil EC of the clay soil (alkaline soil) (Table 3) increased by 117–341% with the application of NEBSBCs. Whereas the addition of BSBCs increased soil EC up to four times in clay loam soil (Table 3).

Table 3 Selected chemical properties of the soils after plant harvest
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3.3 Bioavailability of Major Nutrients from NEBC in Soils

3.3.1 Nitrogen

The application of all NEBCs, BSBC and CF significantly (p < 0.05) increased the concentration of NO3–-N in soil, except in NEChMBC amended soil. Whereas all the NEBCs, BSBC and CF significantly (p < 0.05) influenced NH4+-N in soil. The NO3–-N was significantly influenced by the addition of biochar (1 and 5%), while application of BSBC slightly decreased NH4+-N in soil, and the addition of more biochar significantly increased soil NH4+-N (Table 3). For clay soil, the results of this study revealed that the NO3–-N contents increased by 5 and 41% compared to the control treatment with the application of cow manure and NEBSBC, respectively, after harvesting the canola plants (Table 3). This was 2–37% higher NO3–-N contents than the CF treatment, except the NO3–-N content of chicken manure derived NEBC amended soil was 2.2% lower than CF treatment. In contrast, the CF treatment increased only 3% NO3–-N contents in soil. However, the chicken manure derived NEBC decreased the availability of NO3–-N in the soil by 7%. For clay loam soil, the addition of BSBC reduced the NO3–-N content (46–54%) compared to the control treatment. In comparison, the NH4+-N content was decreased by 25.6% with the application of BSBC at 1% rate.

3.3.2 Phosphorus

In addition, the soil Olsen P contents significantly increased with the addition of all NEBC and other soil amendments. Moreover, BSBC (1%) and BSBC (5%) showed higher Olsen P content in soil than the treatment that received no biochar. In the present study, for clay soil, the addition of NEBCs increased Olsen P by 16–30 times higher than the control treatment. The Olsen P of BSBC amended soils was increased up to four times at a higher dose (5%).

3.3.3 Potassium

The application of biochar had less effects on available soil K, and CF had significant effect on exchangeable K in clay soil (Table 3). The results of the present study revealed that the available K increased by 2.8–25% in NEBCs amended clay soil. The application of BSBC at a high dose (5%) increased the available K content by 105% in clay soil.

3.4 Effects of NEBC on Soil DHA

The effects of adding different types of biochar and CF on DHA in soil are shown in Fig. 1. For the clay soil, among the NEBCs, 1% NEBSBC amendment had a slight increase in DHA, but the 1% NECMBC and NEChMBC amendment did not show any positive response of soil DHA (Fig. 1a). Conversely, the application of NEBCs derived from cow manure, chicken manure and pristine BSBC at 1% led to decreases in DHA by 5.3%, 51.7% and 7.5%, respectively, compared to the control soil. Nonetheless, these DHA levels were 64–221% higher than the CF-amended soil, which experience a decrease of 70.5% compared to the control. For the clay loam soil, the application of BSBC at 5% showed a significant effect on DHA in soil (p < 0.05), while the addition of 1% BSBC decreased DHA in the soil where canola plant was grown (Fig. 1b). For clay loam soil, the high dose of BSBC enhanced DHA by 36% compared to the control treatment that received no biochar or CF.

Fig. 1
figure 1

Effects of biochar on soil dehydrogenase activity. a clay soil; and (b) clay loam soil. Data points represent mean ± SD (n = 3). Mean values denoted by different letters are significantly different (Tukey’s HSD, p < 0.05). Control = without biochar; CF = chemical fertilizer; BSBC = biosolid biochar; NEBSBC = nutrient-enriched biosolid biochar; NECMBC = nutrient-enriched cow manure biochar; and NEChMBC = nutrient-enriched chicken manure biochar

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3.5 Effects of NEBC on Shoot and Root Length

The influence of selected NEBC types (NEBSBC, NECMBC and NEChMBC), BSBC and CF on the growth of canola plant in clay and clay loam soils are presented in Figs. 2 and 3, respectively. Incorporation of NEBC had a significant impact on canola shoot length (p < 0.05), but no positive effect on the root length in clay soil (Fig. 2b). The highest shoot length was found in canola grown in clay soils amended with NEBSBC, NECMBC and NEChMBC applied at a 1% rate (w/w), while the lowest shoot length was obtained in unamended control soils and CF-amended clay soils. For clay soil, all three NEBCs significantly increased the shoot length of canola plants by 67–84% (p < 0.05) compared to the control treatment. Moreover, the addition of BSBC at 1 and 5% improved shoot length by 33 and 40%, respectively, compared to the treatment without biochar and CF. However, the root length of the canola plant was reduced by 5.5–19.3% in NEBC treatments compared to the control in clay soil. The high dose (5%) of BSBC and CF also decreased root length by 5.3% and 8.4%, respectively (Fig. 3b).

Fig. 2
figure 2

Effects of nutrient-enriched biochar on shoot and root length of canola plant grown in clay soil. a shoot length; and (b) root length. Data points represent mean ± SD (n = 3). Mean values denoted by different letters are significantly different (Tukey’s HSD, p < 0.05). Control = without biochar; CF = chemical fertilizer; BSBC = biosolid biochar; BSBC = biosolid biochar; NEBSBC = nutrient-enriched biosolid biochar; NECMBC = nutrient-enriched cow manure biochar; and NEChMBC = nutrient-enriched chicken manure biochar

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Fig. 3
figure 3

Effects of biosolid biochar on shoot and root length canola plant grown in clay loam soil. a shoot length and (b) root length. Data points represent mean ± SD (n = 3). Control = without biochar; BSBC = biosolid biochar; BSBC = biosolid biochar

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For the clay loam soil, the canola plant did not show any significant response to NEBC and CF except for the application of BSBC at 5% that increased root length (Fig. 3b). The highest root length was found in canola grown in the clay soils amended with 1% BSBC (Fig. 2b). The lowest overall shoot length occurred in the clay soils with 1% NEBC (Fig. 2a). In clay soil, 1% BSBC increased root length to 14.5 cm, compared to 12.6 cm for the control. In contrast, NEBSBC, NECMBC and NEChMBC, CF (1%) and BSBC (5%) decreased root length compared to the control. For clay loam soil, no significant response was observed for canola plants with 1% and 5% BSBC.

3.6 Effects of NEBC on Crop Biomass Yield

The influence of NEBC along with BSBC and CF on the above-ground biomass (AGB) and below-ground biomass (BGB) yield of canola plants grown in clay soil and clay loam soils are presented in Figs. 4 and 5, respectively. Application of NEBCs significantly enhanced the AGB yield of canola (p < 0.05) as shown in Fig. 4a. While there was no statistically significant impact on below-ground biomass (BGB) yield, treatments with NEBSBC and NECMBC exhibited a modest increase in BGB yield compared to the control (Fig. 4b). For clay soil, the dry shoot biomass yield increased by two to four folds more than the control treatment, 9.4–69.5% higher than the CF treatment. The application of NEBCs also improved dry root biomass by 7.5–65.8% compared to the control soil by 9.8–69.2% more than CF amended soil. Moreover, BSBC (at 1% and 5% rate w/w) application also increased shoot biomass and root biomass yield by 57.7–130.6% and 20–33%, respectively, compared to the control soil. For clay loam soil, the addition of BSBC (at 1% and 5% rate w/w) significantly (p < 0.05) increased the dry shoot biomass yield (35.6–81.2%) and dry root biomass yield (90.6–101.7%) compared to the control treatment (Fig. 5).

Fig. 4
figure 4

Effects of nutrient-enriched biochar on dry biomass (shoot and root) yield of canola plant grown in clay soil. a shoot biomass; and (b) root biomass. Data points represent mean ± SD (n = 3). Mean values denoted by different letters are significantly different (Tukey’s HSD, p < 0.05). Control = without biochar/chemical fertilizer; CF = chemical fertilizer; BSBC = biosolid biochar; BSBC = biosolid biochar; NEBSBC = nutrient-enriched biosolid biochar; NECMBC = nutrient-enriched cow manure biochar; and NEChMBC = nutrient-enriched chicken manure biochar

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Fig. 5
figure 5

Mean dry biomass yield of canola plant grown in clay loam soil. a shoot biomass; and (b) root biomass. Data points represent mean ± SD (n = 3). Mean values denoted by different letters are significantly different (Tukey’s HSD, p < 0.05). Control = without biochar; BSBC = biosolid biochar; BSBC = biosolid biochar

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3.7 Effects of NEBC on Shoot Nutrient Concentration

The nutrient concentration in shoot biomass of canola plants grown in clay soil and clay loam soils are presented in Table 4. The application of NEBCs had a significant (p < 0.05) positive impact on the N, P and K concentration in shoot biomass of the canola plant. For clay soil, the mean N concentration (56–74%) in canola shoot increased by three to four folds in NEBCs amended soil compared to the control soil. The application of NEBCs significantly (p < 0.05) increased the mean P (24–38%) and K (46–72%) over the treatment that received no biochar or CF. However, the addition of CF also increased the concentration of N (244%), P (88%) and K (12%) that were higher than the NEBC treatments. The BSBC also showed significant improvement (2–4 times more than the control soil) in the shoot N concentration of canola plants. Similarly, the concentration of P and K was also increased with the addition of BSBC by 18–26% and 36%, respectively, over the control treatment (Table 4). For clay loam soil, the overall N, P, and K concentration in canola shoot was significantly (p < 0.05) higher in BSBC amended soil than the control treatment. The shoot N, P and K concentration increased by 4.7–35.2%, 14–194% and 10.5–35% in BSBC amended soil over the control soil.

Table 4 Nutrient concentration in shoot biomass (g kg−1) of canola plant grown on the soils
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3.8 Effects of NEBC on Nutrient Uptake by Canola

Total N, P and K uptake by canola plants grown on the clay soil and clay loam soils are presented in Table 5. The amendment to the clay soil showed a significant (p < 0.05) effect on canola nutrient uptake for all NEBCs, BSBC and CF-amended soils. For the clay soil, the application of NEBCs increased N, P and K uptake by 9–13, 3–5 and 5–7 folds, respectively, compared to the control treatment. The increase was 9.3–58.2% (N), 9.2–18.8% (P) and 80.6–154.8% (K) higher than CF treated soil. Moreover, the N, P and K uptake by canola plants were two to four times higher in BSBC treatments than in the control soil (Table 5). For the clay loam soil, the addition of BSBC increased nutrient uptake by 42.7–145.6% (N), 55–431.7% (P) and K (51.2–148.8%) over the control (Table 4).

Table 5 Nutrient uptake (g kg−1) of canola plant grown on the soils
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4 Discussion

In this study, biochar did not enhance the germination percentage of canola plants, a finding that aligns with Li et al. (2017), who found that oak sawdust biochar reduced tomato seed germination by 35%. Biochar may not always be suitable as a soil conditioner when it contains undesirable compounds, such as dissolved organic carbon, which can inhibit seed germination (Bargmann et al. 2013; Solaiman et al. 2012). High nutrient concentrations in feedstocks may also result in higher nutrient concentrations in their biochars, potentially affecting seed germination (Gaskin et al. 2008). Since biochar treatment hindered crop germination, as observed in our study and earlier reports, it is advisable to avoid biochar application before sowing of seeds to mitigate any negative effects on germination. For a comprehensive understanding of the reasons for poor germination of seeds following biochar application, future research should consider conducting a thorough characterization of the biochar used.

Additionally, CF application did not significantly influence canola seed germination in clay soil (p < 0.05). Instead of enhancing seed germination, CF demonstrated an inhibitory effect. The germination of seeds further decreased as the fertilizer rate increased. With the increasing rate of fertilizer, germination of seeds decreased further. Onyango et al. (2012) reported similar results, noting a reduction in seed emergence with increased N fertilization due to increased osmotic potential. Mumme et al. (2018) also found that the presence of NH4+ in fertilizer could create phytotoxic effects on seed germination. In this study, urea fertilizer (46% N) had the highest N content, which increased the soil’s EC value, suggesting that NH4+ might be responsible for inhibiting seed germination. Moreover, the high concentration of K in the material (KH2PO4) used in this study could also contribute to low germination percentage, as reported by Mumme et al. (2018). In light of these findings, future studies should explore the specific plant growth stages for the application of biochar and CF to minimize potential negative impacts on seed germination and early growth.

Soil pH plays a crucial role in influencing nutrient availability, which in turn impacts plant growth and yield (Dai et al. 2020). Biochar application has been shown to affect soil chemical properties, such as pH, thus altering nutrient dynamics in soil (Hossain et al. 2020). The slight decrease in soil pH was likely due to the release of acidic materials like carboxylic groups from the biochar (Al-Wabel et al. 2018; Wali et al. 2020). The NEBCs were also acidic (pH: 5.4–6.2) in nature. These results agreed with other previous studies (Al-Wabel et al. 2018; Futa et al. 2020; Videgain-Marco et al. 2020). The NEBCs in this study were acidic (pH: 5.4–6.2) and derived from their respective BSBC produced at a low pyrolytic temperature (300 °C), so the biochar pH did not significantly affect soil pH (Ahmad et al. 2012; Cantrell et al. 2012). The addition of BSBC at 1% and 5% decreased the pH from 6.0 to 5.6 in post-harvest clay loam soil (Table 3). Several studies reported that biochar application increases soil pH (Asfaw et al. 2020; Futa et al. 2020; Hossain et al. 2020; Oreoluwa et al. 2020). On the other hand, despite having a pH of 7.8, the BSBC in this study did not noticeably change the pH values of both soils, possibly due to the soils' high buffering capacity.

Biochar application slightly increased the soil's EC value, which could be attributed to the release of loosely bound nutrients or elements into the soil after applying biochar (Hossain et al. 2020; Wali et al. 2020). Previous studies have also observed increased soil EC values following biochar application (El-Naggar et al. 2018; Li et al. 2018; Oreoluwa et al. 2020). The high K content in NEBSBCs could have contributed to the increased soil EC (Hossain et al. 2020; Wali et al. 2020). Thus, applying BSBC at 1% and 5% rates elevated the EC levels up to 10 times in both soils. Furthermore, incorporating CF into clay soil increased the soil EC by 36 times.

Biochar influences the bioavailability of N in soil by two mechanisms: biotic (i.e., fixation, mineralization, immobilization, denitrification, plant uptake) and abiotic (i.e., sorption, volatilization, leaching). The increased bioavailability of N from biochar can be beneficial for plant growth (Hossain et al. 2020). Soil NH4+-N was also increased by 6–15 times higher than the control treatment, which was 83–93% lower than the CF treatment due to the slow release of N from NEBCs. This indicates that N mineralization from biochar occurs in soils (Zhang et al. 2020). With the addition of BSBC at a 5% rate, the soil NH4+-N increased by 252%. This observation corroborates the finding of previous studies (Arif et al. 2017; Jing et al. 2020; Zhang et al. 2020). However, the N content was decreased in clay soils. This might be due to the high N immobilisation by microbes and low N availability (Wali et al. 2020). These findings align with the previous studies (Dai et al. 2020; Hossain et al. 2020; Wali et al. 2020), indicating that the effects of biochar application on N availability in the soil are not consistent and depend on the rate and types of biochar, as well as soil types (Hossain et al. 2020).

Biochar application generally increases the bioavailability of P in soils in three different ways: directly provides available P in the soil; creates a favourable environment for P-solubilizing bacteria; and delays P adsorption or precipitation in soil ( Hossain et al. 2020; Zhang et al. 2019). These findings align with other previous studies (Chen et al. 2020; Jing et al. 2020; Wali et al. 2020). However, the Olsen P in CF-amended soil was found to be 48 times higher than the control treatment and 38.6–67.5% higher than the NEBCs amended soil due to the slow-release nature of biochar compared to mineral fertilizer (Wali et al. 2020).

Biochar generally increases the bioavailability of K in soil (Hossain et al. 2020). This increase could be due to a high concentration of K in three NEBCs, as reported by (El-Naggar et al. 2018). However, this increase was 64–100% less than the CF treatment. In clay loam soil, the addition of BSBC at a rate of 1 and 5% increased the bioavailability of K by 4–11%. Similar findings were also reported in other studies (Chew et al. 2020; Dai et al. 2020; Wang et al. 2020). However, with the lower dose of BSBC, the available K content was slightly decreased (4.7%) in clay soil. This result was also confirmed by Hossain et al. (2020), who reported that biochar has some negative impact on soil.

The DHA represents the overall microbial activity in soil due to oxidation of organic matter and regulating the nutrient dynamics in soil (Liang et al. 2020; Mierzwa-Hersztek et al. 2020). For clay soil, NEBSBC treatment resulted in 30% and 343% higher DHA than the control and CF treatment, respectively. Moreover, the addition of BSBC at 5% increased the DHA by 89% compared to control treatments, possibly due to low temperature (300 °C) biochar (Beheshti et al. 2018; Mierzwa-Hersztek et al. 2020), the synergistic effect of biochar on microbial activity (Mierzwa-Hersztek et al. 2020), and the protective role of biochar in relation to DHA (Futa et al. 2020). The organic matter decomposition influences the enzymatic activity in the soil, and the addition of biochar increased the organic matter in the soil, thereby stimulated the DHA in soils (Beheshti et al. 2017). The results of this present study correspond with other recent studies (Beheshti et al. 2018; Liang et al. 2020; Mierzwa-Hersztek et al. 2020). Previous studies reported that the impact of biochar on enzymatic activity is not consistent (Beheshti et al. 2018; Song et al. 2018). This might be due to changes in soil properties, grown conditions (Futa et al. 2020) and nutrient depletion over time (Mierzwa-Hersztek et al. 2020). Moreover, the impact of biochar on DHA depends on several factors, such as biochar types (properties, origin), soil type, or environmental conditions (Futa et al. 2020).

Biochar has some unique characteristics such as large surface area, cation exchange capacity, and high porosity and thereby, it can influence plant growth (Ibrahim et al. 2020). Biochar had positive impact on root growth in clay soils. This could be due to the lower fertility of soils (i.e., low N and P content) in this study, but some nutrients were available from biochar (Mukhina et al. 2020). The results of this study are supported by other studies (Ibrahim et al. 2020; Mukhina et al. 2020; Olszyk et al. 2020). However, root growth was adversely impacted in clay-loam soils. This could be due to higher nutrient contents (N, P and K) in NEBCs (Bu et al. 2020; Olszyk et al. 2020) can have a toxic effect on the plant root. This is in agreement with earlier results, as reported by Bu et al. (2020). To gain deeper insights into these contrasting effects, it is crucial to emphasize the importance of thorough characterization of biochar, particularly its physical properties, in future studies. Such characterization can help elucidate how chemical treatments impact the surface properties of biochar, and consequently, their effects on plant growth. Notably, meta-analyses and reviews have shown that adding biochar to crops can have both positive and negative effects on crop growth (Jeffery et al. 2011; Kavitha et al. 2018). The inconsistencies are due to differences in soil properties (Haider et al. 2017), biochar characteristics (Omondi et al. 2016) and crop type (Kavitha et al. 2018; Kuppusamy et al. 2016).

The nutrient-rich feedstock, such as manures and biosolids, used to produce biochar have great potential to improve plant productivity in nutrient-deficient soil (Dai et al. 2020). Successful plant growth depends on adequate plant nutrient supply, such as N (Olszyk et al. 2020). NEBCs containing manure and biosolids have been shown to improve dry biomass yield, possibly due to their N-stimulating effect (Clough et al. 2013) and general nutrient-enhancing effect (Agegnehu et al. 2005) on canola plants (Table 2). Recent studies have shown that biochar enhances soil physical and hydraulic properties (Zhang et al. 2020), thereby promoting both root growth (as depicted in Figs. 3 and 4 of this study) and nutrient absorption capacity (Haider et al. 2017; Xiang et al. 2017). These findings are further supported by other studies (Chen et al. 2020; Mukhina et al. 2020).

The increase in shoot nutrient concentration following biochar application has been attributed to improved N utilization efficiency (Chen et al. 2020; Purakayastha et al. 2019). The positive impact of NEBCs on shoot nutrient concentration is consistent with previous studies, although Zhang et al. (2020) reported a decrease in shoot nutrient concentration following biochar application. On the other hand, the addition of CF led to increased nutrient concentration in plants, likely attributed to the slow release of nutrients from the NEBCs (Hossain et al. 2020). Biochar can directly or indirectly increase plant nutrient uptake by various mechanisms, such as increasing soil pH, accumulating organic forms of nutrients, and stimulating soil enzymatic activities (Zhang et al. 2020). Similar to the findings of Zheng et al. (2017), who observed increased N uptake with biochar and fertilizer compared to conventional fertilizer treatment, recent studies have also shown that biochar can increase plant nutrient uptake (Hossain et al. 2020; Mukhina et al. 2020; Videgain-Marco et al. 2020). However, some studies have reported negative or no effects of biochar on plant nutrient uptake (Hossain et al. 2020). Overall, NEBCs have the potential to be used as effective fertilizers for agricultural crop production, especially for the benefit of farmers dealing with low-fertile soils.

5 Conclusions

This study investigated the effects of three nutrient-enriched biochars (NEBCs) and one pristine biosolid biochar (BSBC) on soil properties and canola (Brassica napus L.) growth in two contrasting Australian soils under controlled environment conditions. The NEBC amendments led to a decrease in pH and an increase in electrical conductivity (EC) values in the clay soil, while pristine BSBC also slightly reduced pH and increased EC in the clay loam soil. Moreover, NEBC amendments positively influenced the availability of nitrogen, phosphorus, and potassium, as well as dehydrogenase enzyme activity in the clay soil, while BSBC increased nutrient availability and dehydrogenase enzyme activity in both the clay and clay loam soils.

The application of NEBC resulted in improved canola growth and biomass yield in the clay soil, and BSBC also demonstrated positive effects on canola plants in both soils. Furthermore, NEBC significantly enhanced nutrients recovery in the clay soil, and BSBC also increased plant nutrient recovery in both soils.

Based on these findings, it is evident that applying NEBC at a low dose (i.e., 1% w/w) and BSBC at a relatively high dose (i.e., 5% w/w) can effectively improve soil properties and enhance canola growth. The results of this study underscore the potential benefits of NEBC and BSBC as viable alternatives to chemical fertilizers for promoting soil fertility and increasing crop productivity. The insights obtained from this research pave the way for conducting long-term field trials to assess the efficacy of NEBC at the farm level. By providing valuable implications and showcasing the novelty of these biochar-based fertilizers, this study contributes to the advancement of sustainable agricultural practices.