Synergistic suppression of ammonia volatilization by biochar-compost through soil physicochemical and microbial...

Ammonia (NH₃) volatilization is a major nitrogen (N) loss pathway in salt-affected soils, with losses often exceeding 30% of applied N. It reduces N use efficiency and contributes to the secondary formation of fine particulate matter and aquatic eutrophication. Although biochar and compost have been widely evaluated, their combined effects and action mechanisms in salt-affected soils remain unclear. Herein, we performed soil column experiments using saline soil from the Ningxia Hetao Irrigation District to quantify NH₃ volatilization and assess changes in soil properties and microbial communities under biochar-compost amendment. Results revealed that compared with conventional fertilization (CKU), 1% biochar-compost fertilization (BCOFL) reduced cumulative NH₃ volatilization by 30.82% and decreased biomass-normalized NH₃ flux by 35.24%. Biochar-compost treatments consistently decreased soil pH during both basal and topdressing stages, and the pH remained significantly lower than that under CKU at multiple sampling timepoints. At maize maturity, compost-based amendments improved soil aggregation, increasing the abundance of >0.25-mm water-stable macroaggregates by 74.45%-102.64% and the mean weight diameter (MWD) by 15.77%-45.09% compared to those under CKU. BCOFL usually outperformed the corresponding 1% compost-only treatment (OFL). Sequencing revealed significant community shifts, with the abundance of Firmicutes being higher under BCOFL than under OFL. In addition, Firmicutes abundance was positively correlated with MWD and the total N content (p < 0.05). Predicted N-fixing groups were 57.28% and 85.52% more abundant under BCOFL than under OFL and CKU, respectively. By providing stable microhabitats (a higher MWD), biochar-compost treatment increased the abundance of N-retaining and N-fixing microbes, which indirectly reduced the available ammonium nitrogen pool for volatilization. Structural equation modeling identified decreased pH and increased MWD as primary drivers in suppressing NH₃ volatilization. Overall, biochar-compost treatment is a practical strategy for mitigating NH₃ losses and improving N retention in saline-alkali agroecosystems.