Key Takeaways & Executive Findings
- •• • Rice straw biochar (S) increased soil nitrate nitrogen by 148.74%–152.68% versus control, significantly enhancing nitrification; combined with Bacillus subtilis (SJ) raised average net nitrogen mineralization rate by 77.28%–99.38%. • • Rapeseed straw biochar (Y) decreased nitrate nitrogen by 51.66%–57.61% and reduced net nitrogen mineralization rate by 82.07%–84.73% when combined with inoculant (YJ), indicating a suppressive effect on nitrification. • • Biochar amendments S and Y increased microbial biomass nitrogen (MBN) by 2.02- and 2.20-fold over control, respectively; combined treatments (SJ, YJ) further boosted MBN by 103.26%–149.44% relative to single biochar. • • YJ treatment (4% rapeseed straw biochar + 5 mg/kg Bacillus subtilis) is the optimal amendment for dryland soil, reducing inorganic nitrogen loss risk while enhancing microbial nitrogen activity, as evidenced by lower nitrate leaching potential and higher MBN.
Abstract
Biochar and microbial inoculants are widely used for agricultural soil amendment. To investigate the effects of different straw biochars and Bacillus subtilis inoculant, applied individually or combined, on nitrogen transformation in dryland soil, a 60-day laboratory incubation experiment was conducted with six treatments: control (CK), 4% rice straw biochar (S), 4% rapeseed straw biochar (Y), 4% rice straw biochar plus 5 mg/kg inoculant (SJ), 4% rapeseed straw biochar plus 5 mg/kg inoculant (YJ), and inoculant alone (J). Results showed that rice straw biochar significantly increased soil nitrate nitrogen content by 148.74%–152.68% compared to CK, enhancing nitrification. Combined application with inoculant further increased average net nitrogen mineralization rate by 77.28%–99.38%. Conversely, rapeseed straw biochar decreased nitrate nitrogen by 51.66%–57.61%, and combined application reduced net nitrogen mineralization rate by 82.07%–84.73%. Treatments S, Y, SJ, and YJ promoted microbial biomass nitrogen (MBN) synthesis, with S and Y increasing MBN by 2.02- and 2.20-fold over CK, respectively. Combined treatments further increased MBN by 103.26%–149.44% relative to single biochar treatments. These findings indicate that biochar type governs nitrification and net nitrogen mineralization, while combined application exerts synergistic effects on MBN. For comprehensive dryland soil improvement, YJ treatment is optimal, reducing inorganic nitrogen loss risk and enhancing microbial nitrogen activity.
1. Introduction
Dryland soils constitute approximately half of China's arable land, yet their low moisture content and poor structure lead to significant nutrient loss, particularly nitrogen, through runoff and leaching. Over 50% of applied nitrogen fertilizer is lost annually, causing economic waste and environmental issues such as eutrophication. Conventional soil amendments, including biochar and microbial inoculants, have been explored individually, but their combined effects on nitrogen cycling remain poorly understood, especially the differential impacts of biochar feedstock type.
This study addresses the bottleneck by systematically comparing rice straw biochar (high silica, alkaline) and rapeseed straw biochar (high lignin, acidic) with and without Bacillus subtilis inoculation. The experimental protocol quantifies inorganic nitrogen dynamics, net mineralization rates, and microbial biomass nitrogen under controlled incubation, providing mechanistic insights into how biochar source and microbial synergy can be harnessed to optimize nitrogen retention and reduce environmental losses in dryland agriculture.
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ZHENG Tao, CHEN Yuqing, HUANG Xianhuai, YU Xiongsheng, ZHOU Xiaoyu, LIU Yingchao, ZHUANG Xingmei, FANG Zijun, QIAN Jing (2026). Nitrogen Mineralization Effects of Bacillus subtilis Combined with Straw Biochar in Dryland Soil. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202608026
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Frequently Asked Questions
What are the underlying mechanisms by which rice straw biochar enhances nitrification while rapeseed straw biochar suppresses it?
Rice straw biochar likely increases soil pH and provides favorable habitat for nitrifying bacteria due to its high surface area and nutrient content, leading to a 148.74%–152.68% increase in nitrate nitrogen. In contrast, rapeseed straw biochar may release phenolic compounds or induce nitrogen immobilization, reducing nitrate by 51.66%–57.61%. The exact mechanisms require further investigation into microbial community composition and functional gene expression.
How does the combined application of biochar and Bacillus subtilis affect the net nitrogen mineralization rate compared to biochar alone?
For rice straw biochar, combined application (SJ) increased the average net nitrogen mineralization rate by 77.28%–99.38% relative to control, but was lower than biochar alone (S). For rapeseed straw biochar, combined application (YJ) reduced the rate by 82.07%–84.73% compared to control, but mitigated the reduction caused by biochar alone (Y). This suggests that Bacillus subtilis can modulate nitrogen cycling, but the direction depends on biochar type.
What is the practical significance of the observed increases in microbial biomass nitrogen (MBN) for soil fertility?
MBN is a labile nitrogen pool that contributes to plant-available nitrogen over time. Biochar amendments increased MBN by 2.02–2.20 fold, and combined treatments further increased it by 103.26%–149.44%, indicating enhanced microbial nitrogen immobilization and potential for slow-release nitrogen, which can reduce leaching losses and improve long-term soil fertility.
What are the limitations of this study and what further experiments are needed before field application?
This study was conducted under controlled laboratory conditions for a short duration (60 days). Field trials are necessary to validate the effects under variable environmental conditions. Additionally, the impact on plant growth and emissions of nitrous oxide (N2O) should be monitored to assess environmental trade-offs.
How does the choice of biochar feedstock influence the economic viability of this soil amendment strategy?
Rice straw and rapeseed straw are agricultural by-products, making biochar production cost-effective. However, the differential effects on nitrogen cycling mean that rapeseed straw biochar combined with Bacillus subtilis (YJ) is recommended for reducing nitrogen loss, but its lower mineralization rate may require supplemental nitrogen fertilization. Economic analysis should consider biochar production costs, application rates, and potential savings from reduced fertilizer use.
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