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Yangzhou University (2019)

Effects of Extreme Drought and Species Composition on Carbon Exchange in Semiarid Steppe


Titre : Effects of Extreme Drought and Species Composition on Carbon Exchange in Semiarid Steppe

Auteur : 张慧;

Grade : Doctoral Dissertation 2019

Université : Yangzhou University

Precipitation patterns are expected to change in the semi-arid region within the next decades,with projected increasing in extreme drought events.Meanwhile,the timing of extreme drought also shows great uncertainty,suggesting that the timing of drought,especially during growing season,may subsequently impose stronger stress on ecosystem functions than drought itself.Extreme drought can also affect the composition of plant communities in the ecosystem and lead to shrub development in the grassland ecosystem.Shrub encroachment is increasing in density,cover and biomass under extreme drought.However,how the timing of extreme drought will impact on community productivity and carbon cycle is still not clear.Specially,what role does the plant composition play in responsing to extrme drought.In this study,three timing of extreme drought(a consecutive 30-day period without effective precipitation event)experiments were set up separately in early-,mid-and late-growing season in a temperate steppe in Inner Mongolia since 2013.Meanwhile,the constructed plant composition was set as factor including graminoids,shrubs and their mixture(graminoid+shrub)to test the effects of extreme drought.The data,including soil organic carbon(SOC),soil nitrogen(SN),soil active phosphorus(SP),soil water content(SWC),soil temperature(ST)chlorophyll fluorescence parameter(Fv/Fm),ecosystem respiration(Re),gross primary productivity(GPP),net ecosystem carbon absorption(NEE)and aboveground net primary productivity(ANPP)were collected in growing season(from May to September)of 2014 to 2016.In this study,extreme drought significantly decreased SWC during the drought treatment in growing season.Extreme drought decreased maximum quantum efficiency of plant photosystem II(Fv/Fm)under "optimum" value(0.75~0.85)of two dominant species(Lymus chinensis and Stipa grandis).While ANPP kept stable under extreme drought treatments due to the different responses of two dominant species,which brought a compensating effect in relative abundance and biomass.In addition,only early-growing season drought significantly decreased the average Re(P<0.01)and GPP(P<0.01)and depressed net CO2 uptake(P<0.01)than mid-and late-growing season drought.ST and SWC influenced the changes of GPP directly and indirectly through photosynthetic ability of the dominant species by path analysis.Under the factor of plant composition,extreme drought significantly reduced the average soil water content for three years,and the early-growing season drought had the lowest soil water content.Extreme drought had a significant effect on soil total organic carbon,while plant composition had no significant effect on soil organic carbon.Extreme drought and plant composition had no significant effect on soil total nitrogen.Extreme drought had significant effect on soil available p,but species composition and plant composition had no significant effect on soil active phosphorus.Extreme drought significantly influenced NEE(P=0.05),Re(P<0.01),GPP(P<0.01),and interannual effects between extreme drought and plant composition were significant(P<0.01).Early-growing season drought showed the strongest negative effect in carbon sink.Species composition had significant effects on net ecosystem carbon exchange(P<0.01),ecosystem respiration(P<0.01),total primary productivity(P<0.01).Shrub species are the most carbon absorbent.Extreme drought and species composition(F=10.05,P<0.01)had significant effects on ANPP.Mid-growing season drought had the strongest negative effect on biomass,the biomass of grass species was the highest,and the biomass of shrub species was the lowest.Soil water content has a good linear correlation with NEE(P=0.05),Re(P<0.01)and GPP(P<0.01),and soil temperature has a good linear correlation with NEE(P<0.01)and Re(P<0.01).The contribution of Re to GPP during the drought treatment was greater than that out of drought treatment,indicating that the trend of carbon loss was increased in the drought treatment,resulting in the carbon emission of the grassland.Our results indicated that the timing of drought should be considered in carbon cycle models to accurately estimate carbon exchange and productivity of semi-arid grasslands in the context of changing climate,within the effects of plant composition

Mots clés : extreme drought; plant composition; carbon exchange; plant physiology; productivity; semiarid steppe;

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Page publiée le 2 mai 2020