Influence of exogenous N resource on triploid Populus tomentosa leaf decomposition

Litter decomposition is important for nutrient cycle in terrestrial ecosystems. Released nutrients by litter decomposition are critical for particularly fast-growing and high-yield pulp forests. Delays in on-site nutrient release could limit forest productivity. It is therefore vital to accelerate the decomposition of leaf litter for the rapid release of needed nutrients in forest plantations. To determine ways of rapid litter decomposition and nutrient release, the effects of exogenous ammonium nitrogen, nitrate nitrogen and mixed nitrogen on triploid P. tomentosa leaf decomposition were determined in a laboratory study. The results showed significant differences in the rates of decomposition of triploid P. tomentosa leaf under the different nitrogen resources. Leaf decomposition significantly accelerated under exogenous ammonium nitrogen. Compared with the control (27.4%), leaf decomposed rates under exogenous ammonium nitrogen, mixed nitrogen and nitrate nitrogen resources were 46.0%, 30.0% and 28.8%, respectively, after 140 d decomposition. The fitting results with Olson exponential equation showed that the times for 50% and 95% leaf decomposition under exogenous ammonium nitrogen, mixed nitrogen and nitrate nitrogen resources were 175 d, 316 d, 301 d and 781 d, 1 238 d, 1 627 d, which were shortened by 49.7%, 9.2%, 13.5% and 52.0%, 23.9%, 14.1%, compared with the control, respectively. There remarkable differences in released main nutrients of litter decomposition under exogenous ammonium nitrogen, mixed nitrogen and nitrate nitrogen resources. K content decreased with increasing degree of decomposition, and was not influenced by exogenous nitrogen resource. However, N and P release was influenced by exogenous nitrogen resource. While the accumulation time of N and P shortened from 21 to 7 days, the degree of N accumulation dropped from 1.94 (under the control) to 1.44 (under exogenous nitrogen) times of initial N concentration. The degree of P accumulation also dropped from 2.98 (under the control) to 1.98 (under exogenous nitrogen) times of initial exogenous P concentration. This suggested that exogenous nitrogen improved on-site productivity via accelerated leaf litter decomposition and N, P release, which in turn shortened the decomposition cycle of triploid P. tomentosa leaf litter.