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

Abstract

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.