Different forms of nitrogen deposition show variable effects on soil organic nitrogen turnover in a temperate forest

Abstracts The mineralization of soil organic nitrogen (N) catalyzed by N-hydrolyzing enzymes plays a major role in ecosystems N cycling. However, the responses of this biochemical process to N deposition, particularly different N types, remain unclear. Here, a field experiment was conducted in a temperate forest to investigate how different types of N deposition influences the interaction between soil organic N fractions and N-hydrolyzing enzymes, i.e., protease, N-acetyl-β-glucosaminidase, urease and amidase in the 0–10 cm and 10–20 cm layers. Four treatments including nil N (CK), inorganic N (NH4NO3, IN), organic N (urea and glycine 1:1, ON) and mixed N (inorganic and organic N 7:3, Mix-N) deposition were included. We found no effects of IN deposition on N-hydrolyzing enzymes activities and soil organic N fractions. In contrast, ON and Mix-N deposition significantly increased urease activity in surface soil and protease, urease, and amidase activities in subsoil. Mix-N deposition resulted in the greatest increase in hydrolysable ammonia-N (85%), labile organic N (76%) and stable organic N (101%), followed by ON deposition. Moreover, structural equation modeling revealed that amidohydrolases (urease and amidase) was actively involved in the conversion of labile organic N to ammonia-N, which was promoted by ON and Mix-N deposition but showed no response to IN deposition. In summary, mixed inorganic and organic N deposition favors the accumulation and subsequent enzymatic turnover of organic N in forest soils. Our findings suggest that different components in N deposition should be considered when examining N deposition effects on ecosystem N cycling.