Soil organic matter controls biochar–oil cake effects on inorganic nitrogen availability and mineralization kinetics in volcanic ash soils

Abstract Understanding nitrogen (N) mineralization is essential for accurately estimating N availability from various soil amendments, thereby minimizing nutrient losses and reducing environmental impacts. A laboratory leaching-incubation experiment was conducted to assess nitrogen (N) mineralization potential (N 0 ) and kinetics in volcanic ash soils with varying soil organic matter (SOM) contents (Low: 11.7 g kg − 1 ; Medium: 66.1 g kg − 1 ; High: 134 g kg − 1 ), and to evaluate the effects of co-applying biochar with oil cake (OC) on soil N dynamics. Oil cake was applied at a rate of 200 kg N ha − 1 in combination with biochar derived from satsuma mandarin pruning branches from Jeju orchards at 0%, 0.2%, 1%, and 2% (w/w) rates. Results showed that SOM level significantly influenced biochar effects on N 0 and N release. Using a first-order kinetic model, N 0 was highest in High SOM soils, followed by Medium and Low SOM soils, whereas the 2% biochar treatment consistently resulted in the lowest N 0 across all SOM levels. In Low SOM soils, inorganic N availability (NO 3 − -N + NH 4 + -N) decreased by 4.7%, 8.0%, and 12.9% with 0.2%, 1%, and 2% biochar additions, respectively, compared to the 0% biochar treatment. In contrast, biochar did not significantly affect N availability in High SOM soils. Overall, biochar moderated N mineralization by enhancing N immobilization and reducing the mineralization rate constant, thereby mitigating nitrate (NO 3 − -N) leaching in agroecosystems. The effect was more pronounced in Low SOM soils, indicating that biochar application should be tailored to soil properties and environmental conditions to minimize NO 3 − -N losses and improve N-use efficiency.