Response of evapotranspiration and CO2 fluxes to discrete precipitation pulses over degraded grassland and cultivated corn surfaces in a semiarid area of Northeastern China

The effects of precipitation pulses on evapotranspiration (ET) and CO2 fluxes were evaluated over different land surfaces in a semiarid area using eddy covariance technique. ET responded to rain events throughout the growing season, but the degree of responses varied among seasons. In spring and fall, ET slightly responded to precipitation pulses, because the precipitation amount in the rainfall event was small and very discrete. As the rainy season started in early summer, responses of ET to precipitation pulses significantly increased. In late summer, the intensity of responses in ET was weakened because the soils retain moisture by the high-frequency rainfall events and the antecedent soil moisture. Regarding CO2 exchange, in early summer, the net CO2 uptake slightly increased at the cultivated corn site after rain pulses, while the degraded grassland site experienced comparatively greater responses. In late summer, the cultivated corn site exhibited more negative CO2 flux after rain than the grassland site, and the responses of net CO2 uptake to rain pulses were much stronger than those in early summer. The soil moisture content and vegetation cover are the key factors in determining the responses of ET and CO2 fluxes to precipitation pulses in these two semiarid ecosystems.