Inhibition effects of N deposition on soil organic carbon decomposition was mediated by N types and soil nematode in a temperate forest

Abstract Increasing nitrogen (N) deposition may alter soil organic carbon (SOC) decomposition, thereby strongly affecting SOC storage in terrestrial ecosystems. Its specific influence may depend on the different types of N deposition and soil nematodes. However, little is known about how N deposition and soil nematodes affect the SOC cycle process. To address this issue, we evaluated the effects of different types of N deposition on SOC decomposition under the conditions of applying nematocide or not in a temperate forest. Soils collected from the simulated N deposition forest for 5 years were incubated in the presence and absence of soil nematocide at 15 °C for 150 days. N deposition suppressed soil C cycle processes, such as SOC decomposition and soil enzyme activities, and caused the accumulation of labile SOC, which depended on N types. A mixture of inorganic and organic N (MN) deposition had the highest suppression of SOC decomposition at 31.5%, followed by organic N (ON) deposition (24.4%) and inorganic N (IN) deposition (19.8%), thereby suggesting that inhibition effects of N deposition on SOC decomposition based on a single IN or ON source are underestimated. Nematocide application stimulated SOC decomposition, with the highest in MN (19.5%), followed by IN (13.5%), ON (11.2%), and control treatment (4.6%). The stimulation effect of SOC decomposition by soil nematode exclusion also depended on N types. N deposition and soil nematode exclusion had no interactive effect on SOC decomposition. These results imply that atmospheric N deposition favors the increase of C stocks in soil by reducing the SOC loss, and that N types should be considered during assessment of N deposition effects on soil C cycle processes.