Nitrogen additions stimulate litter humification in a subtropical forest, southwestern China

Despite the importance of nitrogen (N) deposition for soil biogeochemical cycle, how N addition affects the accumulation of humic substances in decomposing litter still remains poorly understood. A litterbag experiment was conducted to assess the potential effects of N addition (0 kg·N·ha −1 ·year −1 , 20 kg·N·ha −1 ·year −1 and 40 kg·N·ha −1 ·year −1 ) on mass remaining and humification of two leaf litter ( Michelia wilsonii and Camptotheca acuminata ) in a subtropical forest of southwestern China. After one year of decomposition, litter mass was lost by 38.1–46.5% for M . wilsonii and 61.7–74.5% for C . acuminata , respectively. Humic substances were declined by 12.1–23.8% in M . wilsonii and 29.1–35.5% in C . acuminata , respectively. Nitrogen additions tended to reduce mass loss over the experimental period. Moreover, N additions did not affect the concentrations of humic substances and humic acid in the early stage but often increased them in the late stage. The effect of N addition on the accumulation of humic substances was stronger for C . acuminate litter than in M . wilsonii litter. Litter N and P contents showed positive correlations with concentrations of humic substances and fulvic acid. Our results suggest that both litter quality and season-driven environmental changes interactively mediate N impacts on litter humification. Such findings have important implications for carbon sequestration via litter humification in the subtropical forest ecosystems experiencing significant N deposition.