Objective Litter decomposition is a critical process in the cycling of carbon (C) and nitrogen (N) within terrestrial ecosystems and is significantly influenced by global nitrogen deposition. Northeast China, as a prominent temperate forest region, provides an important setting for studying litter decomposition under nitrogen addition. Investigating the decomposition of both species-specific and mixed litter in temperate coniferous and broadleaved mixed forests under nitrogen enrichment is essential for predicting how temperate mixed forests may respond in terms of carbon balance and nutrient cycling under future nitrogen deposition scenarios.

Method This study was conducted in an artificial mixed forest located in Shulan City, Jilin Province. A nitrogen addition experiment was established in 2021, including three nitrogen treatment levels: control (CK, 0 kg/(ha·a)), low nitrogen (LN, 50 kg/(ha·a)), and high nitrogen (HN,100 kg/(ha·a)). In August 2023, a litterbag decomposition experiment was carried out to simulate in situ decomposition. The experiment involved litter from Pinus koraiensis, Acer pictum, Tilia amurensis, Fraxinus mandshurica, as well as a mixture of litter from all four species. Litterbags were retrieved at 30, 60, and 300 days to determine the remaining dry mass and the carbon and nitrogen contents of the decomposing litter.

Result After 300 days of decomposition, the litter decomposition rates of the different tree species followed the order: F. mandshurica > T. amurensis > mixed litter > A. pictum > P. koraiensis. Under different nitrogen addition treatments, at 30 days, nitrogen addition promoted the decomposition of P. koraiensis litter but inhibited that of T. amurensis and F. mandshurica; at 60 days, nitrogen addition inhibited the decomposition of T. amurensis litter; at 300 days, suppression of T. amurensis decomposition was observed only under the low nitrogen (LN) treatment. After 300 days of decomposition, nitrogen addition had no significant effect on the carbon retention of all litter types except mixed litter. However, it enhanced nitrogen release from T. amurensis, F. mandshurica, and mixed litter. Except for F. mandshurica, significant differences in the response of the C/N ratio to nitrogen addition were observed in P. koraiensis, A. pictum, T. amurensis, and mixed litter. In addition, a significant positive correlation was found between litter mass remaining and its C/N ratio, highlighting the litter C/N ratio as a critical determinant of decomposition dynamics. Under different nitrogen addition levels, the effects of mixed litter on decomposition rate were all non-additive. Specifically, mixed litter showed antagonistic effects under CK and HN treatments, while a synergistic effect was observed only under the LN treatment.

Conclusion Significant differences exist in litter decomposition rates among different afforestation tree species in Northeast China, with varying response patterns to nitrogen addition. The decomposition of T. amurensis and F. mandshurica litter showed certain inhibitory effects under nitrogen addition. Under low nitrogen conditions, mixed litter was able to mitigate the negative impacts of nitrogen addition on the decomposition of single-species litter.