Nitrogen Addition Enhances Soil Nitrogen Mineralization Through an Increase in Mineralizable Organic Nitrogen and the Abundance of Functional Genes

Atmospheric nitrogen (N) deposition resulting from human activities has significantly increased reactive N levels in terrestrial ecosystems, which is acknowledged as a primary driver of global change. The response and mechanism of soil organic nitrogen (SON) in N deposition are unclear, despite its critical role as a source and constituent of N. This study aims to investigate SON components and the involvement of soil microorganisms in response to N deposition. It also aims to predict changes in soil N dynamics by accumulating data. N deposition was simulated for 3 years (0, 30, 60, and 90 kg N ha⁻¹ year⁻¹). We examined SON components, the abundance of N-cycle functional genes (NFGs), soil enzyme activities, and N transformation through field surveys and indoor cultivation in a Pinus massoniana plantation with classified Luvisols soil. Acid-hydrolyzable N levels increase with N addition, particularly in mineralizable components, namely acid-hydrolyzable amino acid N and ammonia N. This increase correlated with total N and microbial biomass, leading to a positive impact on N hydrolase activities. Additionally, it results a higher relative abundance of NFGs, which serve as indicators of net N transformation. Net N mineralization increased significantly by a factor of 2.97–4.30 times with N addition compared with the control. Simulated N deposition resulted in an increase in mineralizable SON and NFGs, resulting in higher N availability. However, N addition raises concerns regarding potential organic matter losses and phosphorus limitation. Therefore, long-term monitoring is essential for areas with high N deposition.