Vermicompost maintains topsoil fertility by reducing NH3 volatilization and improving 15N/NO3- retention in a saline-alkali soil

Nitrogen (N) loss is a major limiting factor affecting agricultural productivity in saline-alkali soils, with ammonia (NH₃) volatilization and N leaching being the main sources of N loss. In this study, the dynamics of NH₃ volatilization were measured using the open static chamber method (sponge sampling), alongside the distribution of ¹⁵N and NO₃^--N concentrations in layers, in a 30-cm soil column experiment with vermicompost addition after incorporation of ¹⁵N-labeled urea in the upper layer (0–10 cm) of a saline-alkali soil. Destructive sampling was conducted on days 20 and 60 of the column experiment to investigate the influencing factors of NH₃ volatilization and ¹⁵N/NO₃^- retention, respectively. The results showed that the addition of vermicompost to saline-alkali soil decreased cumulative NH₃ volatilization by 45.1%, decreased the ¹⁵N concentration in the bottom layer (20–30 cm) by 17.1%, and increased the ¹⁵N concentration in the upper soil by 48.7%. Vermicompost regulated the abundances of amoA, amoC, and nxrA genes, which can decrease NH₃ volatilization by converting substrate NH₄⁺ to NO₃^-. Additionally, Ca²⁺ adsorption is enhanced (increased by 6.2%) by increasing soil cation exchange capacity (increased by 20.6%), thus replacing the adsorption of Na⁺ (decreased by 13.8%) and decreasing the desorption of NH₄⁺. Vermicompost enhanced the adsorption of NO₃^- by increasing Ca²⁺ and Mg²⁺ and decreasing Cl^- by 30.4% in the upper soil. This study concluded that vermicompost addition can inhibit N loss by reducing NH₃ volatilization and improving ¹⁵N/NO₃^- retention in saline-alkali soils.