Ecosystem Carbon Exchange Under Warming And Precipitation Enhancement In Kobresia Pygmaea Meadow In Northern Tibet

Recently, regional and global climate change has a great impact on terrestrial ecosystems，thecomposition, function and organization of ecosystems were influenced. Grassland ecosystem is a mainpart of terrestrial ecosystem and plays an important role in global carbon cycle. Northern Tibet hasextremely severe climate and weak ecosystem stability, so climate change results in variation of thepattern, process and function of ecosystem extremely easily. Kobresia pygmaea meadow is the mostrepresentative grassland type in northern Tibet. In recent years, Northern Tibet was experiencedwarming process, and its precipitation also increased. It must be cause the ecosystem variation inKobresia pygmaea meadow. The research of carbon exchange under warming and precipitationenhancement could help to determine the effect of climate change on the carbon budget function ofKobresia pygmaea meadow; clear the impact of climate change on the ecosystem productivity,biodiversity; and monitor the mechanism of response to climate change in the grassland ecosystem.In this study, we used the open top chamber (OTC) to elevate the temperature and combined withgrowing season precipitation enhancement to simulate the climate change in Kobresia pygmaeameadow in Northern Tibet. Four treatments were set up in our study, such as Control (CK), Warming(W), Limited Warming (LW) and Limited Warming&Precipitation Enhancement (LWP, increase theprecipitation by20%). We measured the soil temperature, soil moisture, air temperature, biomass andbiodiversity. The components of ecosystem carbon exchange, such as net ecosystem exchange (NEE),ecosystem respiration (ER), soil respiration (SR), gross primary productivity (GEP), ecosystemevapotranspiration (ET) and Water use efficiency (WUE) were measured to analyze the impact ofclimate change on ecosystem carbon exchange. The main conclusions were as follows:(1) In the growing season, the soil temperature at5cm depth increased by1.02℃but nosignificant change below the15cm in OTC. The W plot was significantly affected by warming; LW andLWP were not significantly changed under warming condition. In the W treatment, the soil moisture at5cm depth was decreased; but warming could not affect the soil moisture in other treatment. The meanraise of air temperature was1.91,1.53and1.37℃in the W, LW and LWP, respectively.(2) In OTC, the plant biomass, coverage and height were increased significantly. The importantvalue of Poa was increased markedly; variation in other species was not obviously; thus, we can saythat the Poa was more sensitive to warming than other species. The Shannon-weiner index wasincreased under warming; the Simpson index and the E. Pielou index were not signicantly changed.(3) Warming and precipitation enhancement could strongly promote the ecosystem carbonexchange. The GEP, ER and NEE were increased significantly in the OTC. With the increasing oftemperature, the ET was significantly strengthened in Kobresia pygmaea meadow, which led to thedecrease of WUE in the OTC, especially in the W and LW treatment at the beginning of the growingseason.(4) The temperature and moisture were significantly impact the components of ecosystem carbon exchange, such as GEP, ER and NEE. The components of ecosystem carbon exchange had significantcorrelation with soil temperature and moisture except GEP and soil temperature, ER and soil moisture.The increasing of temperature and moisture plays a positive role in ecosystem carbon exchange. Thebiomass of grass and sedge exist significant correlation with ecosystem carbon exchange, especiallyGEP and NEE. Thus, the warming caused the photosynthetic production increase, and mostly assignedto the grass and sedge plants.(5) Warming and precipitation enhancement could promote the SR, and the increasing rate washigher than ER, led to the SR/ER was reached51%, higher than CK (36%). SR was closely correlatedto soil moisture; the temperature sensitivity was weaker than ER. Soil temperature sensitivity of ER andSR showed a rising trend under warming.(6) There was a linear correlation between the biomass/biodiversity and air temperature; but thebiomass/biodiversity was not significantly correlated to soil temperature and moisture under warmingand precipitation enhancement conditions. The relationship between biomass and biodiversity werecomplex, the results vary in different index. The correlation curve between species diversity index andbiomass was a unimodal distribution, and between the evenness index and biomass was a U-shaped. Itwas also showed that the dominant role of grass and sedge plants in the Kobresia pygmaea meadowecosystem would be further strengthened under warming condition in the future.Our results showed that grass and sedge plants growth was promoted by warming and precipitationenhancement. Photosynthesis, respiration and gross primary productivity were significantly increasedwhich will contribute to the carbon storage of Kobresia pygmaea meadow ecosystem, and increase theecosystem carbon sequestration capacity.