| Under the background of global climate change,nitrogen deposition has dramatically increased worldwide,which affects not only the environment for human survival,but also significantly affects the key process of carbon budget.Litter decomposition play an important role in nutrient cycle,carbon flow and the balance of global carbon in terrestrial ecosystem.Despite the importance of nitrogen?N?deposition for soil biogeochemical cycle,how N addition affects nutrient release and the accumulation of humic substances in decomposing litter still remains poorly understood.A litterbag experiment was conducted to assess the potential effects of N addition(0 kg·N·ha-1·year-1,20kg·N·ha-1·year-11 and 40kg·N·ha-1·year-1)on mass remaining,nutrient release and humification of two leaf litter?Michelia wilsonii and Camptotheca acuminata?in a subtropical forest of southwestern China.The results are as follows:1.Litter composition was significantly affected by tree species.After one year of decomposition,litter mass was lost by 38.1-46.5%for M.wilsonii and 61.7-74.5%for C.acuminata,respectively.Litter decomposition constant that k value ranged from 0.45 to1.28 between two species,and were decreased with increasing N addition concentration for M.wilsonii,but were low-N addition>control>high-N addition in order for C.acuminate.2.C remaining of two tree litter leaf gradually were reduced during the experimental period.In general,the C remaining of M.wilsonii was higher than that C.acuminate.Litter N and P of M.wilsonii showed a pattern of enrichment-release,while those of C.acuminate kept in releasing during the decomposition.After a year of decomposition,C remaining,N remaining and P remaining for M.wilsonii were 4.9-32.4%?67.6-105.6%and 52.4-107.9%,respectively,for C.acuminate were 11.7-17.2%?30.0-44.4%and24.7-57.4%.3.N addition and interaction of species×N addition dramatically affected litter decomposition rate.In generally,regardless of initial quality,higher-N addition inhibited litter decomposition rate and chemical?C,N and P?releasing process in both species.However,lower-N addition showed opposite degradation pattern on between species,while accelerated high-quality substrate?C.acuminata?nutrient releasing,but inhibited in low-quality substrate?M.wilsonii?.4.The concentrations of humic substances,humic acid and fulvic acid of litter exhibited significant differences between two tree species.The effect of N addition on the accumulation of humic substances was stronger in C.acuminate litter than in M.wilsonii litter.After 1-year field incubation,the concentrations of humic substances,humic acid and fulvic acid of litter were reduced by 12.1-23.8%,3.9-34.3%and 23.4-31.9%for M.wilsonii,respectively;and by 29.1-35.5%,12.0-22.4%and 43.8-48.5%,respectively,for C.acuminate.5.The effect of N addition on the litter humification was significantly different with the decomposition process.N additions did not affect humic substances,humic acid and fulvic acid in the early stage but often increased them in the late decomposition period,suggesting that N additions are favorable to the humification in decomposing litters.With the advance of the humification process,N additions may favor transformation from fulvic acid to humic acid.Such conversion could,to some extent,make humus structure within the litters more stable.Our results suggest that both litter quality and decomposition time interactively mediate N impacts on litter humification.Such findings have important implications for carbon sequestration via litter humification in the subtropical forest ecosystems experiencing significant N deposition. |
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