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, Rh) 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 (Re), 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, Re, and GPP during the growing season, followed by photosynthetically active radiation and air temperature (Ta). Daily Re was mainly controlled by air temperature during the non-growing season, when Re accounted for only ~17% of the annual Re due to winter temperatures. Growing season plant respiration (Rp) was the most important source of carbon emissions from the maize field, with aboveground plant respiration being the major part of Rp. Rh accounted for ~60% of total soil respiration. Only ~60% of the annual GPP was lost as Re, 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.