Effects of simulated nitrogen deposition on the decomposition of leaf litter and release of nutrients in a cold temperate coniferous forest in the Jiaozi Snow Mountains National Nature Reserve in southwest China

The decomposition of litter is a key process of nutrient cycling in terrestrial ecosystems. It is an important part of the global carbon budget and deeply affected by the global atmospheric N deposition. However, the effects of different forms of N addition on litter decomposition and nutrient release are unclear in a cold temperate coniferous forest in a subtropical Chinese plateau. Three N sources (NH₄)₂SO₄, NaNO₃, and NH₄NO₃ were used in the gradient N deposition method. Each N source was divided into four treatments, which included the CK (control 0 kg N·hm⁻²·a⁻¹), low N (low-N 5 kg N·hm⁻²·a⁻¹), medium N (medium-N 15 kg N·hm⁻²·a⁻¹), and high N (high-30 kg N·hm⁻²·a⁻¹). The results showed that the average residual rates of dry matter in the leaves litters treated with (NH₄)₂SO₄, NaNO₃, and NH₄NO₃ were 41.78 %, 48.56 %, and 38.78 % after two years of decomposition, respectively. After two years, the litter decomposition rates of low and medium NH₄NO₃ treatments were the quickest compared with the control, while those of the high and low treatments of NaNO₃ were the slowest. Under the same N deposition conditions, the litter decomposition rates of low N treatments were higher than high N treatments. The order of litter decomposition rates was NH₄NO₃ > (NH₄)₂SO₄ > NaNO₃. N deposition promoted the release of nutrient element C in the litter leaves but slightly increased the contents of N and P. The changes in P over time were more complex than those of C and N. Greater amounts of C, N, and P were released in litter leaves than those that were enriched. These results indicate that treatment with N can promote the decomposition of leaves in the litter leaves and thereby release C, N, and P as nutrients in a cold temperate coniferous forest on a subtropical Chinese plateau.