Nitrogen addition, drought and mixture effects on litter decomposition and nitrogen immobilization in a temperate forest

Litter decomposition serves an important role in maintaining nitrogen (N) availability within forest ecosystems. However, the interactive effects of exogenous N, drought, and litter quality and mixing on N immobilization during decomposition remain unclear. The aim of this study was to assess the effects of litter quality, reduced precipitation, N addition, and their interactions on litter mass loss and N immobilization. This field study analyzed the effects of N addition and decreased precipitation on the decomposition rates and associated N immobilization of four types of litter: Quercus mongolica (QM), Tilia amurensis (TA), Pinus koraiensis (PK), and a mixture (MIX) of all three. The chemical quality of the MIX was prepared in a 4:3:3 (mass) ratio of PK, TA, and QM litters. Litterbags were placed in an N addition and precipitation manipulation forest field and collected after 92, 154, 365, 457, and 874 days. Decomposing litter residues were characterized for mass loss and N content to assess N immobilization. The addition of N had no significant effect on litter decomposition under both precipitation conditions, but a reduction in precipitation significantly depressed litter decomposition. The increases in N immobilization with N addition depended on the litter type and decomposition period. Precipitation reduction had significant effects on N immobilization and enhanced the magnitude and duration of N immobilization in decomposing litter, and both of which can be increased by N addition. The results indicate that the litter species is the major regulator that controls mass loss and N immobilization. Furthermore, the MIX treatment did not show non-additive effects on mass loss but did exhibit some weak synergistic effects on N immobilization. Our results suggest that decomposing litters could help to sequester N depending on the litter identity and water regime in temperate forest ecosystems.