Effects of compost and biochar co-application on nitrogen leaching and carbon dynamics in soil-water-plant systems

Compost and biochar are increasingly recognized as complementary soil amendments capable of enhancing fertility and mitigating nutrient losses. This study evaluated yard-waste compost and wood-derived biochar applied individually and in mixtures at 100% compost, 100% biochar, 75% compost with 25% biochar, and 50% compost with 50% biochar to a low-organic matter soil on sloped mesocosms. Compost addition significantly improved plant growth, biomass, and microbial activity, attributed to enhanced nitrogen availability and labile organic carbon inputs. In contrast, biochar alone inhibited plant growth but effectively reduced nitrogen leaching compared to other treatments. Co-application of compost and biochar balanced these responses, promoting vigorous vegetation establishment while minimizing nutrient export. The findings indicate that a combined amendment approach can simultaneously improve soil fertility, structure, and water quality protection, especially when the soil organic matter increases from compost is limited to approximately 2% and nitrogen parameters are maintained within optimal ranges (total N ≥ 2000 mg/kg; available N ≥ 50 mg/kg; C:N ratio 15:1-23:1). These results emphasize the need for site-specific amendment strategies, considering soil organic matter, slope, and nutrient availability.