Increases in substrate availability and decreases in soil pH drive the positive effects of nitrogen addition on soil net nitrogen mineralization in a temperate meadow steppe

Abstract

Nitrogen (N) deposition is projected to increase over the coming decades, with consequences on soil N transformation. Soil N mineralization is an important process regulating soil N availability. However, it is unclear how the responses of soil N mineralization to increasing N deposition are controlled by biotic and abiotic factors. Here, we examined the effects of increasing N addition rates on soil net N mineralization, net nitrification, and N-cycling functional gene abundance in a temperate meadow steppe after six years treatments. We found that N addition significantly increased the rates of soil net N mineralization and nitrification. Nitrogen addition rates were positively correlated with the abundance of ammonia-oxidizing bacteria amoA gene, but negatively correlated with that of other N-related functional genes. The N-induced enhancement of net N transformations were predominantly driven by the enhanced quantity and quality of soil substrates, but not by N-related functional genes. Decreased soil pH also contributed to the enhancement of net N transformations. Our findings highlight the importance of soil substrate availability and soil pH in driving grassland soil N transformation processes under the scenario of increasing N deposition.