**4. Effect of biochar amendment on soil fertility**

Soil fertility refers to a soil's ability to provide essential plant nutrients for sustained crop productivity [175]. Modern agricultural practices often lead to soil degradation and nutrient depletion [175]. Biochar addition improves soil properties by increasing organic carbon and crucial mineral nutrients like nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur [176]. However, biochar's nutrient composition depends on raw material source and pyrolysis conditions, which may not always meet diverse soil requirements [177]. Biochar also enhances nutrient retention and utilization in soil. After incorporation, nitrate and ammonium nitrogen retention increases significantly [178]. Additionally, biochar treatment leads to substantial increases in grain biomass and nitrogen and phosphorus utilization efficiency compared to using nitrogen fertilizer alone [45]. Research on biochar production and applications is rapidly expanding [179, 180]. While carbon, nitrogen, and phosphorus cycling influences soil microbial activity, other essential elements like sulfur, potassium, magnesium, and calcium are crucial for soil fertility and quality [181, 182]. Biochars rich in exchangeable base cations support plant growth [183], and a strong correlation exists between oilseed rape yield and soil potassium content [184]. Comprehensive assessment of soil fertility considers interconnected interactions, with integrative indicators like soil pH, electrical conductivity, and cation exchange capacity providing a more holistic understanding [181, 182].
