| Human activities have significantly altered the global nitrogen cycle in the last several decades such as combustion of fossil fuels,intensive agriculture and stock breeding,which has doubled the natural rate of N fixation.Chronically elevated N inputs to the terrestrial ecosystem have influenced litter decomposition.Human activities such as combustion of fossil fuels,intensive agriculture and stock breeding have significantly altered the global nitrogen cycle in the last several decades.Litter decomposition play a crucial role in soil formation,the availability of nutrients to plants and microorganisms in forest ecosystem.Because of the differences in environmental conditions of different tree species,litter quality or decomposition phases,the effects of N input on litter decomposition can be positive or negative.Litter decomposition may have positive,negative or no effects in response to enhanced N supply.There are many inconsistent hypotheses about those phenomena,and need further research about their mechanisms.The background of N deposition has increased remarkably in many regions.A field experiment was conducted with annual nitrogen additions at four levels,it can predict nutrient return of rare tree species in response to the increased nitrogen deposition,and provide reference for the effects of nitrogen deposition on litter decomposition.It also can provide theoretical basis for the protect of rare tree species and rare tree speacies,and foe the tree species in coping with atmospheric nitrogen deposition.A field simulated experiment was conducted to study the decomposition of dead branches of Taxus chinensis var.mairei,Tsuga tchekiangesis,Tsuga longibracteata Cheng,Cephalotaxus fortumeiHook,f,Fokienia hodginsii, Cryptrom eria fortunei,Metasequoia glyptost roboides Hu et Cheng in response to increased nitrogen deposition.Treatments were designed as N0(0 kg·hm-2·a-1),N1(50 kg·hm-2·a-1),N2(100 kg·hm-2·a-1),N3(150 kg·hm-2·a-1),with three replicates in each treatment.The results show:The dead branches remaining rate of Tsuga tchekiangesis was estimated to be 70.475%～73.955%,with 95% decomposition time of 11.534a,11.633a,10.485a,12.359a,N2 increased the dead branches decomposition,but the high levels of N deposition(N3) was found to inhibit the litter decomposition to some extent.The dead branches remaining rate of Tsuga longibracteata Cheng was estimated to be 81.775%～83.59%,decomposition constant K for the above treatments was estimated to be 0.1933,0.1886,0.1937 and 0.1798,with 95% decomposition time of 15.499a,15.882a,15.468a and 16.658a,N3 inhibited the dead branches decomposition. The dead branches remaining rate of Cephalotaxus fortumeiHook,f was estimated to be 71.245%～72.94%, decomposition of constant K of the above treatments was estimated to be 0.2950,0.2997,0.3307 and 0.3629,with 95%decomposition time of 10.156a,9.932a,9.058a and 8.252a,N3 increased the dead branches decomposition. The dead branches remaining rate of Fokienia hodginsii was estimated to be 80.075%～82.34%,with 95% decomposition time of 15.578a,15.296a,15.027a,14.929a,N2 and N3 increased the dead branches decomposition.The dead branches remaining rate of Cryptrom eria fortunei was estimated to be 72.66%～74.465%,decomposition of constant K in the above treatments was estimated to be 0.3122,0.3141, 0.3365 and 0.3055,with 95%decomposition time of 9.595a,9.537a,8.904a and 9.805a,N2 increased the dead branches decomposition.The dead branches remaining rate of Metasequoia glyptost roboides Hu et Cheng was estimated to be 80.745%～81.49%,decomposition of constant K in the above treatments was estimated to be 0.2181,0.2162,0.2042,0.2062,with 95%decomposition time of 13.734a,13.855a,14.674a,14.529a,N2 and N3 increased the dead branches decomposition.The order of the 7 species decomposition is Tsuga longibracteata Cheng>Fokienia hodginsii>Metasequoia glyptost roboides Hu et Cheng>Taxus chinensis var.mairei>Tsuga tchekiangesis>Cryptrom eria fortunei>Cephalotaxus fortumeiHook.f.The range of carbon initial contents of different tree species was 44.81～53.39%,at the end of the experiment, the size order of release contents of carbon was Cryptrom eria fortunei>Tsuga longibracteata Cheng>Tsuga tchekiangesis>Cephalotaxus fortumeiHook.f>Fokienia hodginsii>Taxus chinensis var.maire i>Metasequoia glyptost roboides Hu et Cheng.After one year of dead branches decomposition,respectively,for NO,N1,N2 and N3,the concentrations of carbon of different tree species showed significant difference among nitrogen treatments except Tsuga tchekiangesis.Conclusively,nitrogen deposition were found to inhibit carbon release of branches of these tree species.The range of nitrogen initial contents of different tree species was 2.537～8.509 g·kg-1,the order was Metasequoia glyptost roboides Hu et Cheng>Taxus chinensis var.mairei>Cephalotaxus fortumeiHook,f> Fokienia hodginsii>Tsuga tchekiangesis>Tsuga longibracteata Cheng>Metasequoia glyptost roboides Hu et Cheng.The concentration of nitrogen in dead branches were found to increase significantly with the doses of nitrogen additions,for NO,N1,N2,N3,showed the significant difference among different treatments.After 1 year, nitrogen element of Tsuga tchekiangesis,Tsuga longibracteata Cheng and Cryptrora eria fortunei were all enriched,NO treatment increased 9.5%,34.7%,46.7%,inspectively.The other four tree species were all released, NO treatment decreased by 14.4%,31.6%,29%,35.2%.P of the seven rare tree species were showing the release of the state,and the size order of the release was Cephalotaxus forturneiHook,f>Fokienia hodginsii> Metasequoia glyptost roboides Hu et Cheng>Cryptrom eria fortunei>Tsuga tchekiangesis>Taxus chinensis var.mairei>Tsuga longibracteata Cheng.The release rate was between 16～83%.There was a similar variation tendency of P contents among different tree species under four nitrogen trentments,Cephalotaxus forturaeiHook.f and Fokienia hodginsii showed significant differences under various treatment conditions,but the other five species showed insignificant.K element released directly,and the release rate was more than the other major elements.Taxus chinensis var.mairei,Tsuga tchekiangesis,Tsuga longibracteata Cheng,Cephalotaxus forturneiHook.F and Fokienia hodginsii were significant differences under various treatment conditions. Increasing nitrogen sources can promote the release of K.The initial contents of Ca,Mg were low.Ca element contents in Tsuga tchekiangesis was significantly lower than that in other rare tree species,only 0.055 g·kg-1. After a year's experiment,P of these rare tree species showed the release of the state,except Tsuga tchekiangesis, and release rate between 0.11～0.66%.The range of Mg initial contents of different tree species was 0.24～0.46 g·kg-1,Mg release rates of Taxus chinensis var.mairei,Tsuga tchekiangesis,Tsuga longibracteata Cheng, Cephalotaxus fortumeiHook,f,Fokienia hodginsii,Cryptrom eria fortunei and Metasequoia glyptost roboides Hu et Cheng were 38.2%,40.62%,15.2%,46.67%,11.02%,55.12%,20.84%,respectively.
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