Direct and indirect effects of nitrogen additions on fine root decomposition in a subtropical bamboo forest

As an important contributor to carbon (C) flux in the global C cycle, fine root litter in forests has the potential to be affected by the elevated nitrogen (N) deposition observed globally. However, the direct effects (on current fine root decomposition) and indirect effects (on root quality and subsequent decomposition) of N deposition on fine root decomposition are poorly understood. We conducted a 5-year field experiment in a Pleioblastus amarus bamboo forest in southwestern China. In the first 3 years of the experiment, N-treated sites (0, 50, 150, and 300 kg N ha(-1) year(-1), respectively) and fine roots under two N regimes (0 and 150 kg N ha(-1) year(-1); 0 N-Root/+N-Root, respectively) were prepared. Next, these two fine root treatments were applied to a 2-year decomposition experiment in the N-treated sites under continuous N treatment. Nitrogen additions increased fine root density, concentrations of N, P, and lignin in fine roots, and concentrations of TOC, TN, NH4 (+)-N and NO3 (-)-N in the soil, and decreased the soil pH. The decomposition constant k decreased under N addition treatments, and the decomposition rate of + N-Root was lower than that of 0 N-Root, suggesting that both the direct and indirect effects of N additions on fine root decomposition rates were negative. Both N addition and root substrate changes led to an increase in the residual lignin content during decomposition. Nitrogen additions significantly decreased the loss of C, N, P, K, Ca and Mg during decomposition. The changes in the soil environment and increased root lignin concentration as a result of N additions may be the mechanism underlying the negative direct and indirect effects observed. Elevated fine root biomass input and slower degradation rates may be a potential mechanism explaining the increase in soil TOC and TN under N treatment.