Carbon budget assessment of an irrigated wheat and maize rotation cropland with high groundwater table in the North China Plain

Abstract. Carbon sequestration of cropland has the potential to mitigate global greenhouse gas emissions. To understand such sequestration of an irrigated wheat-maize rotation cropland with high groundwater table in the North China Plain, the carbon budget and its components are estimated with a comprehensive field experiment by combining eddy covariance technique, soil respiration experiment differentiating heterotrophic and below-ground autotrophic respirations, and biometric measurements in a relatively wet year from October 2010 to October 2011. In the experimental period of a whole winter-wheat and summer-maize cycle, the Net Ecosystem Exchange, Gross Primary Productivity, Ecosystem Respiration, soil heterotrophic respiration, below-ground autotrophic respiration and above-ground autotrophic respiration are −437.9, 1078.2, 640.4, 376.8, 135.5 and 128.0 gC m⁻², respectively for wheat season, and are −238.8, 779.7, 540.8, 292.2, 115.4 and 133.2 gC m⁻², respectively for maize season. The experiment allows for estimations of Net Primary Productivity, Net Ecosystem Productivity and Net Biome Productivity. The Net Biome Productivity are 58.8 and 3.9 gC m⁻² for wheat and maize season, indicating that wheat is a carbon sink and maize is close to carbon neutral. However, compensated by the net ecosystem carbon release in two rotation periods, Net Biome Productivity of the whole wheat-maize rotation cycle is 12.8 gC m⁻² yr⁻¹ in the experimental year, indicating this cropland remains a weak carbon sink under the specific climatic conditions and field conditions with a high groundwater table. The cropland has a higher ecosystem carbon use efficiency (CUE) than other terrestrial ecosystems, indicating that the agro-ecosystem is more efficient in harvesting CO₂ from the atmosphere. This irrigated wheat-maize rotation cropland with high groundwater table has higher CUE than other croplands, implying that the cropland management of full irrigation and fertilization promotes carbon accumulation in crops.