Mechanism Of Nitrogen Deposition On Undergrowth And Soil Bacterial Community Diversity In Subtropical Evergreen Broad-leaved Forest

Forest ecosystem is the most important component of the terrestrial ecosystem.Since the industrial revolution,the rapid increase of nitrogen deposition has posed a serious threat to the structure and functions of forest,which has had a negative infiuence on the forest role to the terrestrial.Subtropical evergreen broad-leaved forest occupies a special place in Chinese forest and is also an important part of the global evergreen broad-leaved forests.This study was designed to explore the effect of increasing nitrogen(N)deposition on undergrowth and soil bacterial community diversities across a subtropical evergreen broad-leaved forest,to reveal the response mechanism of nitrogen deposition.In a 4-year simulation experiment of nitrogen deposition in the north subtropical evergreen broad-leaved forest,the following results were obtained.1.Effects of N additions on soil physical and chemical properties.The moisture content(SWC)of surface soil(0-10 cm)showed an increasing trend with N additions.The SWC was increased by 15.43%in High-N treatment in mid-aged forest compared with CK,without significant difference,and was significantly greater in High-N+P(+101.37%)and P(+75.10%)treatments than in CK.In the mature forest,compared with CK,the SWC was significantly increased by 23.96%in High-N treatment,while significantly decreased by10.47%in the High-N+P treatment.In old-growth forest,SWC showed a trend with High-N(+22.06%)>High-N+P(+13.05%)>Low-N(+9.12%)>CK,and no significant differences were observed between N treatments and CK.Soil pH showed a decreasing trend in N and P additions.In mid-aged forest,soil pH decreased significantly compared to CK,by 10.58%in High-N,11.39%in High-N+P and 6.42%in P treatments.In the mature forest,the order of soil pH ranked as CK>High-N(–4.02%)>High-N+P(–6.64%),with no significant difference among the treatments.In the old-growth forest,compared to CK,pH slightly decreased by 0.95%in Low-N treatment,and significantly decreased by 4.02%in the High-N and 6.64%in High-N+P treatments.Compared with CK,dissolved organic carbon(DOC)and nitrogen(DON)contents in High-N treatment increased to a certain extent for different aged forests.The DOC and DON increased by 13.10%and 19.65%in High-N treatment in mid-aged forest,respectively;increased by 5.18%and 10.48%in the mature forest;and in old-growth forest,increased by 17.66%and 5.76%.Soil organic carbon(SOC)content increased significantly in High-N treatment in this study.The changes of soil microbial biomass carbon and nitrogen differed obviously among the stands.2.Effects of N additions on forest growth.N additions promoted the growth of tree diameter at breast height(DBH).The DBH increment was significantly higher(by 70.63%)in High-N+P treatment than P treatment in mid-aged forest.The DBH increments were significantly greater in High-N(+6.75%)and High-N+P(+12.88%)treatments than in CK in the mature forest.In the old-growth forest,the maximum DBH increment was found in High-N+P treatemnt.However,N additions had not significant effect on the increments of stand basal areas for the different aged forests.The results from multivariate analysis indicated that DBH growth was controlled by forest age(F=6.141,P<0.05).3.Effects of N additions on undergrowth diversity.There existed somewhat difference in species composition of the undergrowth for different aged stands,with Shannon-Wiener indices ranged from 1.66 to 2.40.Two shrubs Symplocos sumuntia and Camellia fraternal were the co-existing species in all sampling stands.In the old-growth forest,N additions had no significant impact on Shannon-Wiener indices.Compared with CK(2.40),Shannon-Wiener index declined significantly in High-N(24.58%)treatment in mid-aged forest.In the mature forest,compared with High-N treatment,Shannon-Wiener index increased significantly High-N+P(65.25%)treatment,but no significant difference between CK and High-N+P treatment.The Simpson index(-11.36%)and Alatalo index(-5.41%)were significantly decline in High-N treatment in mid-aged forest.The diversity indices of herb layer did not change significantly in N addition treatments in this age-sequence forest.4.Effects of N additions on foliar ecophysiological trait of major shrubs.The N additions had no significant impact on foliar nutrient contents and their stoichiometric traits for different aged forests.However,N addition had a certain effect on correlations between different elements.Compared with CK,N content was significantly associated with P content in High-N(r=0.503,P<0.01)and High-N+P(r=0.501,P<0.01)treatments,indicating that High-N treatment can improve the efficiency of P absorption use.My results demonstrated that N addition increased foliar N contents in major shrub species.To a certain extent,the N and P additions influenced foliar nutrient stoichiometric traits and specific leaf area of understory shrubs.5.Effects of N additions on soil bacterial community diversity.Soil microbial diversity is not significantly affected nitrogen additions in different aged forests.In High-N and High-N+P treatments,OTUs decreased in mid-aged forest,while increased in the mature forest,and showed different changes in the old-growth forest.In addition,Shannon–Wiener indices decreased in mid-aged forest and increased in old-growth and mature forests.Simpson indices showed slight changes in N addition treatments for the different aged forests.Schao1 indices declined in mid-aged forest and increased in the old-growth forest,and showed different changes in the mature forest.6.Effects of N additions on soil bacterial community composition.Proteobacteria,Acidobacteria and Actinobacteria were the main phyla in different treatments for the age-sequence forests.Bacterial community structure and composition were changed by the different N additions.There existed significantly differences in genus-level OTUs of the following phyla including Proteobacteria,Acidobacteria,Actinobacteria,Planctomycetes,Bacteroidetes,Firmicutes and Candidatus Saccharibacteria in mid-aged forest;In mature forest,the significant differences were found in the follwing 8 phyla,i.e.Proteobacteria,Acidobacteria,Actinobacteria,Bacteroidetes,Candidatus Saccharibacteria,Firmicutes,Verrucomicrobia and Gemmatimonadetes;and 6 phyla such as Proteobacteria,Acidobacteria,Actinobacteria,Firmicutes,CandidatusSaccharibacteriaand Cyanobacteria/Chloroplast in the old-growth forest.7.The results from Principal Component Analysis and Redundancy analysis demonstrated that N additions had little effect on stand growth(R~2=0.105,P=0.582),and had marginal significant effect on undergrowth diversity(R~2=0.203,P=0.051)in these subtropical forests.However,both stand growth(R~2=0.224,P=0.005)and undergrowth diversity(R~2=0.311,P=0.006)were significantly correlated with forest age.In addition,N additions had significant effect on leaf ecophysiological indices(R~2=0.204,P=0.044)and soil bacterial community structure(R~2=0.309,P=0.006)in this study.