StudyonCarbonFluxanditsControllingmechanisms In A Degraded Alpine Meadow And An Artificial Pasture In The Three-River Source Region Of The Qinghai-Tibetan Plateau

To understand carbon budget of artificial and degraded grassland ecosystemsin alpine region, we examined CO2flux and its affecting factors at an Elymus nutans artificial pasture in the Three-River Source Region of the Qinghai-Tibetan Plateau from January1st,2006to31December31st,2008and a degraded alpine meadow in this region from September16th,2006to December31st,2008, using eddy covariance method. The results of the study were summarized as follows:1. The pasture was a weak sink for atmospheric CO2, with a net ecosystem productivity(NEP) of35.2,52.1, and140.0g Cm-2yr-1in2006,2007and2008respectively. The meadow was a source for atmospheric CO2, which released8.4g Cm-2yr-1in2007. However, during2008, integrated NEP of the meadow was43.2g Cm-2yr-1, indicating that it was a weak sink of carbon. One important finding was that the CO2uptake at the meadow was more than that at the pasture for July-August, which may be result from the decreasing of plant diversity at the pasture. Although the inter-annual variability of two ecosystems’carbon budgetwas large, but the average carbon sequestration capacity per year of the pasture was obviously higher than that inthe meadow by78.7g C m-2yr-1.2. Annual ecosystem respiration (Re) in pasturewas460.8,449.5, and403.6g Cm-2yr-1in2006,2007and2008, respectively.Annual Re in meadow was472.8and312.2g Cm-2yr-1in2007and2008, respectively. Annual gross primary productivity (GPP) in pasture was543.3,501.6, and496.1g Cm-2yr-1in2008,2007and2006,respectively. Annual GPPin meadow was in2007and2008was464.4and355.4g Cm-2yr-1). GPP was36.2g Cm-2from September16th,2006to December31st,2006in meadow, respectively. Overall, GPP was higher in pasture than that in meadow.3. Seasonal patterns of the NEE (net ecosystem CO2exchange), Re and GPP were similar at the meadow andpasture. Peak daily sums of NEE, Re and GPP all happened in July and August in both ecosystems. In pasture, the maximum daily NEE was-1.7,-1.8and-2.9g Cm-2day-1in2006,2007and2008, respectively. In meadow, the maximum daily NEE were-2.7and-3.0g Cm-2day-1in2007and2008, respectively. The maximum daily NEE was higher in meadow than that in pasture. In pasture, the maximum daily Re were5.6,4.3and5.0g Cm-2day-1in2006,2007and2008, respectively. In meadow, the maximum daily Re was4.3and3.8g Cm-2day-1in2007and2008, respectively. In pasture, the maximum daily GPP were5.8,4.9and6.0g Cm-2day-1in2006,2007and2008, respectively. In meadow, the maximum daily GPP were5.8and5.9g Cm-2day-1in2007and2008, respectively.4. NEE and Re of the pasture exhibited a diurnal variation similar to that of the meadow during growing seasons (May-September), the hourly maximum NEE of both ecosystems occurred before noon, while the hourly maximum Re occurred around16:00pm. The hourly maximum NEE in the pasture were-6.9,-7.3and-7.9μmol CO2m-2s-1in2006,2007and2008, respectively. The hourly maximum NEE in the meadow were-8.5and-8.0μmol CO2m-2s-1in2007and2008, respectively. The hourly maximum Re in the pasture was5.6,5.0and5.0μmol CO2m-2s-1in2006,2007and2008, respectively. The hourly maximum Rein the meadow was4.9and3.6μmol CO2m-1s-1in2007and2008, respectively. The hourly maximum GPP of both ecosystems appeared from11:00tol5:00. The hourly maximum GPP in the pasture were9.6,10.1and10.9μmol CO2m-1s-1in2006,2007and2008, respectively. The hourly maximum GPP in the meadow were11.2and10.3μmol CO2m-2V-1in2007and2008,respectively.5. The study of root respiration show that soil respiration rate in pasture (2.43μmol CO2m-2s-1) was lower than that in meadow (5.38μmol CO2m-2s-1). The proportion of the root respiration to the total soil respiration was very similar in both ecosystems, and the proportion in the pasture and meadow were67%and70%, respectively.6. The model-derived quantum yield (a) and saturated NEE (Fmax) of PPFD-NEE curve in the pasture and meadow increased with canopy development during the growing seasons. Meanwhile, a and Fmax controlled by soil water content (SWC), air temperature (Ta), and vapor pressure deficit (VPD); The relationship between daytime NEE and Ta can be depicted by a quadratic formula, and the stepwise regression analysis showed that the optimal Ta values of the hourly daytime NEE in the pasture and meadow were13.2℃and12.1℃, respectively; The hourly daytime NEE linear increased with the SWC in the pasture and meadow;The relationship between daytime NEE and VPD can be depicted by a quadratic formula, and the stepwise regression analysis showed that the optimal VPD values of the hourly daytime NEE in the pasture and meadow were0.62kPa and0.60kPa, respectively.7. During the growing seasons, Re in both ecosystems increased exponentially with the temperature, and the apparent temperature sensitivities of ecosystem respiration (Q10) of the pasture and meadow were3.5～4.4and3.0～3.1, respectively. The Q10in meadow obvious lower than that in pasture, which indicated that the pasture will lose more carbon in the context of global warming.8. Path analysis showed that PPFD and Ta had the greatest effect on carbon sequestration capacity during the growing seasons at the meadow and pasture ecosystems.