Effects of nitrogen addition on litter decomposition and nutrient release in two tropical plantations with N2-fixing vs. non-N2-fixing tree species

Atmospheric nitrogen (N) deposition has elevated rapidly in tropical regions where N-2-fixing tree species are widespread. However, the effect of N deposition on litter decomposition in forests with N-2-fixing tree species remains unclear. We examined the effect of N addition on litter decomposition and nutrient release in two tropical plantations with Acacia auriculiformis (AA, N-2-fixing) and Eucalyptus urophylla (EU, non-N-2-fixing) in South China. Three levels of N additions were conducted: control, medium-N (50 kg N ha(-1) yr.(-1)) and high-N (100 kg N ha(-1) yr.(-1)) in each plantation. Initial decomposition rate (k (a) ) for the control plots was faster in the AA plantation than in the EU plantation, but later in decomposition, larger fraction of slowly decomposing litter (A) remained in the former. N addition increased the slow fraction (A), decreasing soil microbial biomass and reducing acid-unhydrolyzable residue (AUR) degradation in the AA plantation. In the EU plantation, however, N additions significantly increased initial decomposition rate (k (a) ) and soil N availability. Furthermore, N addition decreased litter carbon and N release (in the AA plantation), while litter phosphorus release also decreased in both plantations. With ongoing N deposition in future, tropical plantations with N-2-fixing tree species would potentially increase carbon accumulation and nutrient retention in forest floor by slowing litter decomposition.