| In this paper,based on the on-site investigation of the organic carbon process of Kandelia obovata mangrove forests(K)under vegetation restoration project,sea level rise and shrimp pond wastewater discharge and combined with the relationship between organic carbon processes in these ecosystems,three empirical models are established to predict the main organic carbon processes of mangrove forests influenced by main human activities and global changes by using Matlab.These models predict the main organic carbon process of K.obovata mangrove forests at different time scales and can reveal the tendency of these processes which are consistent with other studies.The restoration projects of Kandelia obovata mangrove forests were carried out in Jiulong Estuary,Fujian Province in 1998,1986 and 1962.By 2020,the recovery time serises of these forests in this region are 22 years(22K),34 years(34K),and 58 years(58K),respectively.From 22 to 58 years,the ecosystem organic carbon storage increased by 163.45 t C ha-1,showing an order of 22K<34K<58K,opposite to the order of NPP and the increasement is mainly reflected in the growth of vegetation.The"S" growth curve can better reflect the variation in vegetation biomass during the natural recovery process,and the determination coefficient R2 is 0.9933.The growth of vegetation biomass is slow at long restoration projcet,but the annual yield of litter is relatively stable.Therefore,the proportion of litter on the NPP is increasing with restoration time.Because daily flooding time in each forest plot is similar and short,the decomposition rate of microorganisms determines the proportion of fate of leaf litter and the leaching output is negligible.The amount of crab eating,microbial decomposition and tital output are controlled by litter yield.The half-decomposition period of withered leaves in each forest floor is relatively close,and the average amount of microbial decomposition accounts for about 42.34%of the total withered leaves,and the tide output accounts for 45.48%.Therefore,the tide output in this area is the main destination of litter.Mangrove sedimentation is not only a fate of litter organic carbon,but also has a rate of mineralization emiting CO2 into atmosphere,which is slower than carbon accumulation rate.Therefore,it is believed that the successful restoration of mangrove vegetation can inhibit the increasement of atmospheric CO2 concentration to a certain extent in a long period,especially in the early stage of restoration due to high vegetation growth productivity.The mangorve plots with three different elevations at the Zengying coast in Xiamen represent sea level rise(SLR)80 cm,40 cm,and 0 cm(SLR 80,SLR 40,and SLR 0).Ecosystem organic carbon stock follows the SLR 0>SLR 40>SLR 80 in both the short-term(2020)and longer time scales(2099).However,with time goes by,the difference in organic carbon storage between SLR 80 and SLR 0 increased from 72.63 t C ha-1 in 2020 to 98.87t C ha-1 in 2099,while the difference between the SLR 40 and SLR 0 increased from 26.30t C ha-1 in 2020 to 38.55 t C ha-1 in 2099 in this simulation.However,the soil carbon storage shows a different pattern with the sea level rise and time.In 2020,the order of soil carbon storage is SLR 0>SLR 40>SLR 80,and by 2099,the order is SLR 80>SLR 40>SLR 0.This is due to the gas carbon flux of SLR 40 and SLR 80,which is smaller than SLR 0,so that drcreasing the soil organic carbon export.The change in tidal leaching caused by various daily flooding time in different plots is the main reason for the change of withered leaves fate ratio.The leaf organic carbon output by tidal water in SLR 0 accounts for 36.17%of the total annual leaf litter organic carbon,which is lower than 43.92%at SLR 40 and 49.03%at SLR 80.The ratios of leaching output at SLR 0,SLR 40,and SLR 80 to the total tidal output were 2.86%,14.46%,and 27.49%,respectively.Therefore,the difference in tidal output is mainly reflected in the output of leaching organic carbon.In addition to considering the daily changes of wet and dry conditions in the forest floor,this paper also coupled with the microbial decomposition process.It is concluded that the decomposition effect of microorganisms on litter is still stronger than the leaching effect on the time scale of the year,which is reflected in the rate of leaching to microorganisms decomposition(DOCtide/Cmicrobe),and this value gradually decreases with decreasing flooding time.Although the prolonged flooding environment exacerbates the soil anaerobic environment causing the sediment-atmosphere CO2 flux decreased with sea level rise,this also causes vegetation growth and net primary productivity decline in the early stage.Overall,sea level rise will not be conducive to the functioning of the carbon sequestration potential in the entire ecosystem.On the south of the Jiulong Estuary in Fujian,there are two plots of K.obovata forest that have been affected by shrimp pond aquaculture in 2004(N2)and 2010(N1).In order to measure the intensity of the disturbance,one area that is not directly affected by the sewage is introduced(R).The ecosystem organic carbon storage order under the influence of shrimp pond sewage discharge in 2020 is as follows:406.22 t C ha-1 in N1(short impact time)>296.40 t C ha-1 in R(Reference)>259.29 t C ha-1 in N2(longterm affected),because the disturbance intensity and discharge time of shrimp pond wastewater affect the subsequent growth pattern of vegetation.The nitrogen element(N)in the aquaculture wastewater accelerates the biogeochemical process after entering the mangrove sedimentation,so that the CO2 flux in N1 and N2 regions is about twice that of in the R.The linear regression equation can reflect this well.The flooding time of N1,N2 and R forest land was similar so that the output of organic carbon tidal water,microbial decomposition and crab consumption were mainly controlled by the annual yield of litter.The rate of microbial decomposition amount to litter production in R,N1,and N2 are 36.19%,36.12%,and 32.94%,respectively,which is closely related to the semi-decomposition time of leaf litter.Appropriate aquaculture wastewater to mangrove forests at a certain time point not only promotes vegetation growth,but also accelerates the biogeochemical cycle of carbon. |
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