Response of carbon and nitrogen release to simulated nitrogen deposition in natural evergreen broad-leaved forests in a rainy area in Western China

Over the last several decades, human activities have increased the nitrogen input in terrestrial ecosystem, and the alteration of the N cycle has profoundly affected the cycles of many other elements, especially carbon (C). Litter decomposition is an important ecosystem process that plays a key role in the balance of global carbon and nitrogen cycles. To investigate the effects of simulated nitrogen (N) deposition on litter decomposition, a one-year field experiment on litter decomposition following N addition treatments was conducted in a natural evergreen broad-leaved forest in a rainy area of Western China. Beginning November 2013, we conducted a field experiment using the litterbag method that included four treatments: 0, 50, 150, and 300 kg N hm− 2 a− 1 as the control (CK), low (L), medium (M), and high (H) nitrogen deposition, respectively. Every 15 days, NH4NO3 was added to the N-treated plots. The results indicated that after one year of decomposition, the remaining rate in each treatment was between 54.71% and 63.52%, L, M, and H rates were 4.18%, 6.53%, and 8.81% higher, respectively than the CK. N deposition significantly increased the remaining rate and inhibited the decomposition of leaf litter in this ecosystem because of the addition of N. Greater increases in N strengthened the inhibition effect. N addition significantly increased the concentration of carbon and nitrogen in leaf litter. The decomposition coefficient of C and N were in the order k(CK) > k(L) > k(M) > k(H), although N was released faster. The time for 95% C decomposition of foliar litter increased by 0.92-2.20 a from 4.09 a (T95% of CK) because of N addition, and that for N decomposition increased by 0.64-1.22 a from 3.73 a. After decomposing for 1 a, the C remaining rates for L, M, and H were 6.00%, 9.89%, and 14.11%, respectively, higher than CK, whereas the N remaining rate was 4.13%, 6.75% and 10.08%, respectively. Thus, nitrogen addition significantly increased the C and N remaining rates and significantly inhibited the release of carbon and nitrogen. The L, M, and H treatments increased the C/N ratio of the leaf litter by 3.33%, 5.40% and 6.38%, respectively, suggesting that the M and H treatments significantly increased the C/N ratio. Nitrogen treatments weakened the correlation between the mass remaining and C and N remaining, but strengthened the correlation between mass remaining and the C/N ratio. The correlation coefficient of mass remaining and C and N remaining was reduced by N deposition, whereas the C/N ratio was increased. Under simulated N deposition, C remaining was still a good indicator of litter decomposition relative to the N remaining and C/N ratio.