Nitrogen deposition effect on forest litter decomposition is interactively regulated by endogenous litter quality and exogenous resource supply

Background and aims Atmospheric nitrogen (N) deposition affects litter decomposition. However, how endogenous litter quality and exogenous resource supply alter the N deposition effect on litter decomposition and deposited N immobilized by microbes remains unclear. Methods We conducted a laboratory experiment to examine how the N deposition effect on litter decomposition varies with endogenous litter quality (needle litter with higher C/nutrients, low quality litter versus leaf litter with low C/nutrients, high quality litter) and exogenous resource supply (five treatments: N addition alone; N plus non-N nutrient and/or carbon addition; control) using a 15 N tracing method. Results Nitrogen deposition increased the % mass and % N remaining across the decomposition process. Adding non-N nutrients increased the N deposition effect on % mass and % N remaining in the decomposing high quality litter but not in the low quality litter. Moreover, the % P remaining was increased in the low quality litter but was decreased in the high quality litter under N deposition. However, adding N and non-N nutrients together increased the % P remaining in both decomposing litters. The immobilized exogenous 15 N abundance (IEN) was much higher in the decomposing low quality litter than high quality litter. For low quality litter, resource addition treatments affected IEN, but their effects depended on decomposition stages. For high quality litter, carbon addition alone generally increased IEN across the 720 days. Conclusions Nitrogen deposition effect on litter decomposition could be altered by exogenous resource supply, but the pattern ultimately depended on endogenous litter quality. Nitrogen deposition generally suppressed the litter decomposition and non-N nutrients addition enhanced the inhibition effects of N deposition on litter decomposition, especially of high quality litter, while lower quality litter tended to immobilize more exogenous deposited N. Thus, the magnitude of both non-N nutrient availability and litter quality needs to be taken into consideration when assessing the effects of N deposition on litter decomposition.