Carbon budget of a rainfed spring maize cropland with straw returning on the Loess Plateau, China

Assessing the carbon budget of rainfed agricultural ecosystems is a vital component in the process of estimating the global carbon balance. We used eddy covariance techniques combined with soil respiration measurements to estimate the carbon budget of a rainfed spring maize field where straw returning was practiced, on the Loess Plateau, China, during 2012-2014. Carbon fluxes and their components (except heterotrophic respiration, R_h) exhibited single-peak seasonal patterns, and linear relationships were found between daily gross primary productivity (GPP) and net ecosystem exchange (NEE), and between daily GPP and ecosystem respiration (R_e), with goodness of fit value of 0.96 and 0.85, respectively. The green leaf area index was the most important factor controlling seasonal variations in daily NEE, R_e, and GPP during the growing season, followed by photosynthetically active radiation and air temperature (T_a). Daily R_e was mainly controlled by air temperature during the non-growing season, when R_e accounted for only ~17% of the annual R_e due to winter temperatures. Growing season plant respiration (R_p) was the most important source of carbon emissions from the maize field, with aboveground plant respiration being the major part of R_p. R_h accounted for ~60% of total soil respiration. Only ~60% of the annual GPP was lost as R_e, resulting in an average annual net CO₂ uptake of 509gCm^-2. Taking into account carbon exported (483gCm^-2) and carbon imported (10gCm^-2), the average annual net biome productivity was 37gCm^-2, indicating that the spring maize field with straw returning on the Loess Plateau was a weak carbon sink.