Seasonal and Annual Variations of CO2 Fluxes in Rain-Fed Winter Wheat Agro-Ecosystem of Loess Plateau, China

To accurately evaluate the carbon sequestration potential and better elucidate the relationship between the carbon cycle and regional climate change, using eddy covariance system, we conducted a long-term measurement of CO2 fluxes in the rain-fed winter wheat field of the Chinese Loess Plateau. The results showed that the annual net ecosystem CO2 exchange (NEE) was (-71.6+/-5.7) and (-65.3+/-5.3) g C m(-2) y(-1) for 2008-2009 and 2009-2010 crop years, respectively, suggesting that the agro-ecosystem was a carbon sink (117.4-126.2 g C m(-2) yr(-1)). However, after considering the harvested grain, the agro-ecosystem turned into a moderate carbon source. The variations in NEE and ecosystem respiration (R-eco) were sensitive to changes in soil water content (SWC). When SWC ranged form 0.15 to 0.21 m(3) m(-3), we found a highly significant relationship between NEE and photosynthetically active radiation (PAR), and a highly significant relationship between R-eco and soil temperature (T-s). However, the highly significant relationships were not observed when SWC was outside the range of 0.15-0.21 m(3) m(-3). Further, in spring, the R-eco instantly responded to a rapid increase in SWC after effective rainfall events, which could induce 2 to 4-fold increase in daily R-eco, whereas the R-eco was also inhibited by heavy summer rainfall when soils were saturated. Accumulated R-eco in summer fallow period decreased carbon fixed in growing season by 16-25%, indicating that the period imposed negative impacts on annual carbon sequestration.