Elevation of biochar application as regulator on denitrification/NH₃ volatilization in saline soils

Denitrification and NH₃ volatilization are the main removal processes of nitrogen in coastal saline soils. In this incubation study, the effects of wheat straw biochar application at rates of 0, 2, 5, 10 and 15% by weight to saline soil with two salt gradients of 0 and 1‰ on denitrification and NH₃ volatilization were investigated. The results showed that the denitrification rates with 2, 5 and 10% biochar amendments decreased by 25.26, 33.07 and 17.50%, respectively, under salt-free conditions, and the denitrification rates with 2 and 5% biochar amendments under 1‰ salt conditions decreased by 17.74 and 17.39%, respectively. However, the NH₃ volatilization rates increased by 8.05-61.73% after biochar application. The path analysis revealed the interactions of overlying water-sediment system environmental factors in biochar-amended saline soils and their roles in denitrification and NH₃ volatilization. Environmental factors in sediment exerted much greater control over denitrification than those in overlying water. In addition, environmental factors exhibited an indirect negative influence on denitrification by negatively influencing the abundance of the nosZ gene. The comprehensive effects of the environmental factors in overlying water on NH₃ volatilization were greater than those in sediment. The NH₄⁺-N content, pH of overlying water and sediment salinity were the main controlling factors for NH₃ volatilization in saline soils. Biochar application effectively regulated the denitrification rate by changing the environmental factors and denitrifying functional gene abundance, but its application posed a risk of increased NH₃ volatilization mainly by increasing NH₄⁺-N, EC and pH in overlying water.