The Influence Of Water Table Decline On Plant Community And Greenhouse Gases Flux In The Zoige Peatland

The Zoige peatland is the largest alpine peatland in the world;it contains huge carbon stock and hence is important to global carbon cycle.High water table is a key factor in maintaining the carbon stock of peatlands.However,thie Zoige peatland is now experiencing water table decline probably due to global warming and human activities.In order to study the effect of water table decline on the plant community and greenhouse gases flux(including CO2,CH4 and N2O)of Zoige peatland,an In situ field experiment was conducted involving three treatments:deep,shallow,and control water tables,which were achieved by experimental drainage with 50 cm,20 cm,and 0 cm deep ditches,respectively.The monitoring period was from 2013 to 2016.Experimental drainage decreased water table height by ca.12 cm and 15 cm(relative to the intact)and soil moisture(at 5cm soil)by 6%and 12%(relative to the intact(45%))on average in shallow and deep water table treatments,respectively,during the experimental years.The study results are shown as follows.(1)Experimental drainage changed the plant community from the hygrophyta community to the myscophyta community.Specifically,after four years experiment,the dominant species in the shallow and deeep water treatment changed from Kobresia Spp.to Anemone trullifolia var.linearis.Meanwhile,typical mescophytes species-Halenia elliptica,Cerastium caespitosum and Koeleria cristat emerged.Water table decline improved the species richness by 14%-21%and diversity by 6.5%-12%.In addition,water table decline significantly affected aboveground plant biomass,showing negative,neutral,and positive effects on aboveground plant biomass(APB)in the first,second,and third and fourth years,respectively.This duration-dependent response of APB can be attributed to the changes in community species composition during the study years as noted above.(2)In the in-situ field experiment,the effect of water table decline on soil respiration varied with season.In the non-growing season(from October to April),the effect of water table decline was non-significant possibly because the low-temperature effect overrode the drainage effect on soil respiration.In the growing season(from May to September),water table decline mostly increased the emission.However,when soil moisture was lower than 25%,water table decline decreased soil respiration.By contrast,in the soil-core experiment,water table decline always increased soil respiration rate in the growing season.Therefore,it could be concluded that soil-core experiments might overestimate the effect of water table decline on soil respiration in nature due to the restriction of controling water table depth and the lack of minicing water table fluctuation.(3)The measurements of ecosystem CO2,CH4 and N2O fluxes using static chamber method revealed that the effects of water table decline on the three types of greenhouse gases were different.Four-year consecutive monitoring revealed that water table decline increased CO2 emissions by 20%and 9%on average in the deep and shallow water table treatments,respectively,and the increase only occurred in the growing season.In contrast,water table decline significantly decreased CH4 emissions by 64% and 63%in the deep and shallow water table treatments,respectively,and the decrease only occurred in the non-growing season.However,the effect of water table decline on N2O emissions was statistically non-significant.In addition,the biomass of areabic bacteria and methanotrophs were significantly increased in drained plots,which might be partly responsible for the fluxes of greenhouse gases.In summary,experimental drainage have lead to the succession of the pristine hygrophyta community to the mesophyta community along with the increase of species richness,diversity and productivity;meanwhile,water table decline improved ecosystem CO2 emissions,but inhibit CH4 emission.All above results indicate that water table decline may affect the carbon cycle of Zoige peatland through changing plant community species composition and productivity,as well as the fluxes of greenhouse gases.