Biometeorological effects on carbon dioxide and water-use efficiency within a semiarid grassland in the Chinese Loess Plateau

Semiarid grassland ecosystems play a crucial role in global carbon and water cycles. Considerable attention has been devoted to the effect of biometeorological parameters on ecosystem carbon exchanges in semiarid regions. However, a gap remains regarding carbon and water flux in the Chinese Loess Plateau. In this study, we describe 6 years (2007-2012) of eddy-covariance carbon fluxes (CO2) and water-use efficiency (WUE) over a semiarid grassland site and their biometeorological controls. Interannual CO2 flux variations were analyzed based on a correlation analysis with measured biometeorological factors. The annual net ecosystem CO2 exchange (NEE) was negative (mean net carbon sink) across the entire period, varying from -236.3 (2012) to - 114.2 g C m(-2) year(-1) (2011), with a mean value of - 168.9 g C m(-2) year(-1). The normalized differential vegetation index is the most highly correlated with gross primary production (GPP) and evapotranspiration (ET) across several timescales. The effective precipitation frequency is an important factor influencing CO2 fluxes on an annual timescale, and soil water content was the second most important factor affecting CO2 fluxes on a monthly timescale. On an annual timescale, the semiarid grassland ecosystem WUE increased with the soil drought stress index, with ET decreasing more than GPP as soil drought increased. The monthly mean WUE varied from 0.44 to 2.51 g C kg(-1) H2O, and the monthly WUE plateaued in October. Summer drought-induced leaf senescence caused GPP reduction in weaker summer monsoon years, leading to a notable reduction in WUE. We found obvious transitional characteristics in the response of WUE to soil water stress. We also found a strong positive relationship between the annual NEE and the annual effective precipitation frequency at the study site.