Effects Of Simulated Nitrogen Deposition On Soil Soluble Nitrogen Contents And Nitrogen Mineralization In Midsubtropical Forests Soils

A field simulated nitrogen deposition and litter manipulation experiment was conducted to study the effects of nitrogen deposition and forest litters on soluble nitrogen contents and mineralization in subtropical forests of two natural broad-leaved forests (Cinnamomum chekiangense, CIC and Castanopsis fabri, CAF) and an adjacent coniferous plantation(Cunninghamia lanceolata, CUL). Two N treatments of high N (100kg N-ha-1·yr-1) and low N (30kg N-ha-1·yr-1) and litter disposal (kept, removed) were established in the three forests. Meanwhile, short-period lab culture experiments were also conducted. The main results were as follows:(1) After three days of N addition, soil inorganic nitrogen contents increased significantly under HN treatment in different forest types. Monthly dynamics showed that under HN treatment NH4+-N contents increased more significantly in June and August, but for NO3--N contents in February and November. Soil SON contents of the three stands under HN treatment were increased2.1%～26.7%in CIC,2.6%～45%in CAF and5.3%～133%in CUL, respectively. Besides, SON contents were higher under litter kept treatment than under litter removed treatment, in August than in any other months,and in coniferous forest than in broad-leaved forests. After longer time of N addition, soluble nitrogen contents were of no significant difference among different nitrogen treatments.After three day short-period of N addition, net ammonification rate (NAR), net nitrafication rate (NNR) and net mineralization rate (NMR) were mostly positive, but were mostly negative after an longer periods of91to188days of N addition, and mineralization rates of the three stands in June and August were higher than in the other months.It was showed that the three forests had higher inorganic nitrogen contents in summer and wet seasons, than in winter and dry seasons. Analysis of variance indicated that tree species and nitrogen treatments had significant effects on soil soluble nitrogen content, and that inorganic nitrogen contents in the two broad-leaved forests were higher than in the Chinese fir plantation.(2) Lab experiments showed that adding one form of nitrogen increased significantly soil NH4+-N content (for (NH4)2SO4) or NO3--N content (for KNO3); but when two forms of nitrogen (NH4NO3) applied at the same time, soil NO3--N and NH4+-N contents increased concurrently, featured with a larger increase amplitude for the former when compared with adding nitrate nitrogen(KNO3) and a less increase amplitude for the latter when compared with adding ammomium nitrogen((NH4)2SO4). The effect of different forms of nitrogen added on soil NMR in early stage of incubation (the first six hours) showed KNO3>NH4NO3>(NH4)2SO4. Adding different forms of nitrogen influenced soil NNRs significantly, which were being declined with incubation time, and the largest NNR appeared after two hours of nitrogen application.