Nitrogen addition and plant functional type independently modify soil mesofauna effects on litter decomposition

Soil fauna plays a crucial role in plant litter decomposition. However, if and how increasing nitrogen (N) deposition and plant functional type jointly affect the contribution of soil mesofauna to litter decomposition remains poorly understood. A litterbag experiment with two meshed sizes (0.1 and 2 mm) was conducted to examine the effects of soil mesofauna on litter decomposition of three coexisting functional types of trees (evergreen broadleaf, deciduous broadleaf and coniferous) differing in litter identity (6 tree litter species for each plant functional type). Litterbags were exposed to three N addition levels (0, 20 and 40 kg N ha(-1) yr(-1)) in a subtropical forest of southwest China. Overall, soil mesofauna significantly increased litter mass loss with the most profound effect in evergreen broadleaf litter. Further, the high N addition treatment stimulated the effects of soil mesofauna on litter decomposition, particularly in deciduous broadleaf litter. Soil mesofauna effects were positively correlated with initial leaf litter chemical characteristics, such as carbon and cellulose concentration and C/P ratio. The N effect was greater in low N than high N treatments in the 0.1 mm litterbags, but was similar in the two N addition levels in the 2 mm litterbags. Our results indicate that soil mesofauna effects on litter decomposition at low N addition are higher in evergreen broadleaf than in deciduous broadleaf and coniferous litter, and generally increase with N addition levels, particularly in deciduous litter species. The findings underscore the importance of soil mesofauna and plant functional types for litter decomposition in subtropical forests where N deposition is substantially increasing.