| Forest ecosystem is the main body of terrestrial ecosystem and is the largest carbon pool in the world except for oceans.Since the industrial revolution,the rapid increase of nitrogen deposition poses a serious threat to the structure and function of forest ecosystems,and has great impact on function of forest ecosystems in global carbon cycling.Evergreen broad-leaved forest is a typical zonal vegetation in South China,which is an important part of the global evergreen broad-leaved forest.This study was designed to reveal the effect and response mechanism of nitrogen deposition on carbon storage in this subtropical forest ecosystem.In this experiment,two kinds of nitrogen deposition simulation experiments were designed,high nitrogen(HN,100 kg N·hm-2·a-1)and high nitrogen plus phosphorus(HN+P,100 kg N·hm-2·a-1+50 kg N·hm-2·a-1),and added another control?CK,without addition of nitrogen and phosphorus?.Through a 5-year simulation experiment of nitrogen deposition,the following results were obtained.Effects of different N,P additions on soil physicochemical properties.After 5-year nitrogen deposition experiment,the changes of soil bulk density and moisture showed the same trends as HN>CK>HN+P.However,changes of soil pH,EC and organic matter showed different patterns.Soil pH was CK?4.37?>HN+P?4.34?>HN?4.29?,EC was CK>HN>HN+P,and organic matter appeared HN+P(55.21 g·kg-1)>HN(53.25g·kg-1)>CK(50.50 g·kg-1).In 0-10 cm soil layer,soil TP and N:P ratio were significantly different between the different treatments,and the N:P ratio in different treatment was showed as HN+P?23.66?>HN?21.98?>CK?21.65?.The TC,TN,C:N and C:P ratios were not significantly changed by N and P additions.Effects of different N,P additions on the understory vegetation diversity.The diversity indices of the shrub and herb layers were not significantly different between different treatments?p>0.05?.The diversity indices for the shrub layer were lower in HN treatment than in CK and HN+P treatment.The richness index?S?,Shannon-Wiener index?H?and Piolou evenness index?Jsw?were HN+P>CK,while Simpson index?Simp?and Alatalo evenness index?AL?were CK>HN+P.For the herb layer?including the vines?,most diversity indices were higher in HN and HN+P treatments than in CK,the average values of S,Simp and H was higher in HN than in HN+P treatments,but the other two evenness indices Jsw and AL were higher in HN+P than in HN treatments.In combination of shrub and herb layers,the diversity indices were ranked as CK>HN>HN+P,and there were significant differences among the treatments?p<0.05?except for the richness index.It suggested that the N and P additions could decrease the diversity of shrub layer.Effects of different N,P additions on tree growth.The N,P addition can promote DBH growth,the result of the different treatments were shown as HN+P>HN>CK.After5-year experiment,the stand DBH increased by 1.63 cm in CK,and by 1.74 cm in the HN treatment and by 1.84 cm in HN+P treatment.The DBH increment were significantly different between different treatments?p<0.05?.The N and P additions had little effect on the overstory biomass.After 5 years of N,P treatments,the biomass increment of the arbor layer was not significantly different between treatments.For the single-tree biomass in differnet treatment,the increment ranked as HN+P?75 kg?>HN?57 kg?>CK?55 kg?and the difference was not significant?p>0.05?.Effects of different N,P additions on shrub growth.The result of biomass model fitted by shrub samples showed that the binomial function model and the power function model with parameters of D2H?square of basal diameter×height?and CH?mean of crown diameter×H?were better for biomass estimation of shrubs and the prediction precision was more than 0.8.Based on the model fitted,the increments of shrub biomass for the different treatments showed HN+P(850.65 kg·hm-2)>HN(449.65 kg·hm-2)>CK(247.78kg·hm-2).The foliar biomass,whole plant biomass and aboveground biomass of shrubs were significantly different among the treatments?p<0.05?.However,N and P additions decreased the root/shoot ratio of shrub,the order ranked as CK?0.42?>HN?0.32?>HN+P?0.30?,and the difference between the treatments were not significant?p>0.05?.Effects of different N,P additions on vegetation C storage.The N and P additions were increased shrub layer carbon density.The shrub aboveground carbon density ranked as HN+P(0.62 t·hm-2)>HN(0.33 t·hm-2)>CK(0.21 t·hm-2),and were significantly different among treatments?p<0.05?.The root carbon density of shrub in different N,P treatment showed the same trend as aboveground,with HN+P(0.19 t·hm-2)>HN(0.11t·hm-2)>CK(0.08 t·hm-2).N and P additions had certain effects on carbon density of the arbor layer without significant difference between treatments.The carbon density increments of tree stem,branches and roots were consistent,with trend of HN+P>HN>CK.The carbon density increment of foliage was different,and ranked as HN(1.42t·hm-2)>HN+P(1.35 t·hm-2)>CK(0.97 t·hm-2).The results showed that N and P additions increased the stand carbon density of arbor layer,which was HN+P(18.47t·hm-2)>HN(22.98 t·hm-2)>CK(26.12 t·hm-2),but the difference between treatments were not significant?p>0.05?.Effects of different N,P additions on soil C storage.Soil carbon density in 0-10 cm layer was HN(50.60 t·hm-2)>CK(42.50 t·hm-2)>HN+P(35.54 t·hm-2)for different treatments,and HN treatment increased soil carbon storage was same as the previous studies.Soil carbon density in 10-20 cm changed little by N and P additions,with an average about 25 t·hm-2.The results indicate that the N and P additions has great impact on carbon storage of surface soil and the effect was weakened with soil depth... |
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