Effects Of N, P Additions And Roots On Soil N Mineralization Rates In A Subtropical EvergreenBroad-Leaf Forest

Nitrogen (N) and phosphorus (P) are important nutrient elements that limits net primary productivity (NPP) in forest ecosystem. N mineralization rate is a crucial index for soil N supplying capacity, which determines soil N availability and influence N turnover rate and forest productivity. Human activities are greatly increasing the amount of N and alter the N cycle. Soil microorganisms are the main participants of N mineralization. Roots and its exudates would affect N mineralization rate by affecting microbial activity.In order to study the effects of fertilization on soil N mineralization rate in evergreen broad-leaf forest, four treatments were conducted at the following levels: CK (0 kg N·hm-2·year-1), N (100 kg N·hm-2·year-1), P (50 kg P·hm-2·year-1) and NP (100 kg N·hm-2·year-1+50 kg P·hm-2·year-1). In order to study the effects of root on soil N mineralization rate in evergreen broad-leaf forest, two treatments (CP: Untrenched plot and TP:trenched plot) were conducted. We investigated contents of soil NH4+-N, NO3--N, pH, MBN and calculated the ammonification, nitrification and mineralization rates. The results were as followed:(1) Soil NH4+-N and NO3--N concentrations showed significantly seasonal variations and soil inorganic N was mainly composed of NO3--N. Soil N mineralization rate showed significantly seasonal variations.(2) N addition significantly increased soil NH4+-N and NO3--N concentrations and decreased soil pH and N mineralization rate. P addition significantly decreased soil NO3--N concentration and increased and N mineralization rate. N+P addition significantly increased NO3--N concentration. Linear model analysis showed that the effect ratio of fertilization on soil inorganic N was significantly and negatively correlated with that on soil pH.(3) Soil NO3--N concentration significantly increased in the root exclusion plots. Soil N mineralization rate significantly decreased by 27% and 32% as well as 27% in the root exclusion plots under N and P as well as N+P additions, respectively.These results suggest that N and P additions as well as roots have effects on soil N mineralization rate in evergreen broad-leaf forest. It is useful to us to understand the effects of N and P additions on N cycle.