The effects of simulated deposited nitrogen on nutrient dynamics in decomposing litters across a wide quality spectrum using a 15N tracing technique

Abstract

Nitrogen (N) deposition affects litter decomposition. However, how nutrients, especially deposited N are immobilized and released in decomposing litters with different qualities (C/N and C/P) remains unclear. We conducted a laboratory microcosm experiment with four litter types and a combination of a coniferous and deciduous litter treated with N addition (6 mg 99.99% 15N g−1 litter) and control by measuring N-deposition effect on mass (NDEM), N (NDEN), and P remaining percentages (NDEP), deposited 15N immobilized abundance, and microbial composition and enzyme activities in decomposing litters during two years of incubation. The values of NDEM, NDEN, and NDEP were generally greater for the litters with intermediate C/N and C/P than those with the highest and lowest ratios after 360 days, although these parameters varied among different quality litters before 180 days. Immobilized exogenous 15N abundance by microbes showed an increasing trend with increasing litter C/N and C/P across the whole 720-day period. Both C/N and C/P were generally correlated with decomposition rate, 15N immobilization abundance, the microbial richness, and main enzyme activities in decomposing litters. N-deposition effects on N and P dynamics in decomposing litters varied with their C/N and C/P, generally exerting an unimodal curve at later decomposition stages. Lower quality litter with higher C/N and C/P favoured N immobilization in response to N addition.