Effect of <scp> <i>Nigella sativa</i> </scp> Residue and Its Biochar on Soil Microbial Activity, Carbon Sequestration and Nitrogen Cycling Across Different Soil Types

ABSTRACT This study aimed to assess the effects of black cumin ( Nigella sativa ) residue and its biochar on soil microbial activity, carbon sequestration and nitrogen cycling in three soil types: Alfisol, Entisol and Mollisol. A 120‐day incubation experiment was conducted to evaluate changes in soluble carbon (SC), microbial biomass carbon (MBC), soluble nitrogen (SN), microbial biomass nitrogen (MBN), CO 2 emissions, carbon use efficiency (CUE) and metabolic coefficient (qCO 2 ), calculated to understand microbial functionality and carbon retention. The results revealed that biochar (BC) enhanced long‐term carbon stabilisation, while black cumin residue increased microbial activity and short‐term nutrient turnover. In the BC2 treatment, SC remained stable until day 120 (419.2 mg/kg), suggesting sustained carbon availability. The highest MBC (528 mg kg −1 ) and lowest CO 2 emissions were also recorded under biochar treatments, reflecting improved microbial efficiency. Significant interactions ( p &lt; 0.001) were observed between soil type, treatment and sampling time. These findings suggest that the combined use of biochar and plant residues can enhance both immediate microbial responses and long‐term soil health, offering a sustainable strategy for soil fertility improvement and carbon management across different soil systems.