Effects Of Nitrogen Deposition On Litter Decomposition Of Pinus Armandi

China has become one of the third largest nitrogen deposition areas in the world.The continuous nitrogen deposition will have certain impacts on the function and structure of forest ecosystem.Studying the effects of nitrogen deposition on forest ecosystems is very important for predicting and revealing the material cycling process of forest ecosystems in this area under nitrogen deposition.From May 2019 to February 2021 the field decomposition bag method were used to study the litter decomposition of Pinus armandi forest in Guanyinshan National Nature Reserve of Shaanxi Province.Three plots were set,with 16 subplots in each plot.Three different forms of nitrogen were added:ammonium(NH₄Cl),mixed nitrogen(NH₄NO₃)and nitrate(Na NO₃).There were five nitrogen deposition levels for each of the three different forms of nitrogen,including N1(0.5 g N·m^-2·a^-1),N2(1.25 g N·m^-2·a^-1),N3(2.5 g N·m^-2·a^-1),N4(5 g N·m^-2·a^-1)and N5(10 g N·m^-2·a^-1).One control,named as N0(0 g N·m^-2·a^-1)was also set in each plot.After the start of the experiment,at 1,2,3,5,7,12,15,18 and 21months taken samples regular.The dry matter quality,carbon(C),nitrogen(N),phosphorus(P),cellulose and lignin contents of the litter of P.armandi were measured regularly.The results were as follows:1.The deposition of ammonium and nitrate nitrogen at the N1,N2,N3,and N4 levels promoted the decomposition of P.armandi litter,and inhibited the decomposition of P.armandi litter at the level of N5.Different levels of mixed nitrogen deposition promoted the decomposition of P.armandi litter.In general,mixed nitrogen deposition at N3 level significantly promoted the decomposition of P.armandi litter,while nitrate deposition at N5level significantly inhibited the decomposition of P.armandi litter.2.Under the three forms of nitrogen deposition treatment,the C content of P.armandi litter was the enrichment-release model.Ammonium deposition promoted C release from P.armandi litter at N1,N2,N3 and N4 levels,and inhibited C release at N5 level.Different levels of mixed and nitrate deposition promoted C release from P.armandi litter.the N content of P.armandi litter was the enrichment-release model.The three forms of nitrogen deposition promoted N release from P.armandi litter at N1,N2,N3 and N4 levels,and inhibited N release from P.armandi litter at N5 level.At N1,N2,N3 and N4 levels,the release model of P was enrichment-release.Different levels of ammonium deposition promoted the release of P.Mixed nitrogen and nitrate deposition promoted the release of P.armandi litter at N1,N2,N3 and N4 levels,and inhibited the release of P.armandi litter at N5 level.3.The cellulose residues rates of three forms of nitrogen deposition treatments increased first and then decreased,and the average cellulose residues rates was nitrate>mixed nitrogen>ammonium.There were significant differences in the cellulose residues rates of P.armandi litters with the same form and different levels of nitrogen deposition(P<0.05).There were also significant differences in the cellulose residues rates of among nitrate,ammonium and mixed nitrogen at N1,N4 and N5 nitrogen deposition levels(P<0.05).4.The lignin residues rates of three forms of nitrogen deposition treatments increased first and then decreased,and the average lignin residues rates was nitrate>mixed nitrogen>ammonium.Three forms of nitrogen deposition promoted the degradation of lignin.The lignin residues rates of P.armandi litter with same form while different levels of nitrogen deposition reached significant difference(P<0.05).At the same nitrogen deposition level,there were significant differences in lignin residues rates among nitrate,ammonium and mixed nitrogen deposition treatments(P<0.05).