Carbon budget of a winter-wheat and summer-maize rotation cropland in the North China plain

Crop management exerts a strong influence on the agroecosystem carbon (C) budget. From October 2007 to October 2008, the net C budget of an intensive winter-wheat and summer-maize double cropping system in the North China Plain (NCP) was investigated in a long-term field experiment with crop residues input, using a combination of eddy covariance, crop growth and soil respiration measurements. The objectives were to qualify the annual C budget and to establish the effects of climatic variables and crop management on C budget.The net ecosystem exchange of CO2 (NEE) was partitioned into gross primary production (GPP) and total ecosystem respiration (TER); meanwhile, net primary production (NPP) and soil respiration (SR) were determined to compute autotrophic and heterotrophic respirations. Results showed that the NEE, NPP and SR were 359, 604 and 281gCm-2 in wheat season respectively, and 143, 540 and 413gCm-2 in maize season respectively. Autotrophic respiration dominated TER and was mainly driven by GPP. The net C budget was calculated seasonally based on NPP and considering C input through crop residues and C output through grain harvest. We found the winter-wheat system was a C sink of 90gCm-2; whereas, the summer-maize system was a C source of 167gCm-2. Thus, the double cropping system behaved as a C source of 77gCm-2 on an annual basis, corresponding to an annual average loss rate of nearly 1% in topsoil organic carbon stocks during 2003-2008. Though the season length was 52% shorter for maize (113 days) than that for wheat (235 days), over 55% of the CO2 emissions originated from the warmer and rainy maize season; this implies that the inter seasonal climate variability affected the C flux dynamics mainly and the interaction of soil temperature and moisture is the "single" dominant factor for ecosystem respiration in this area. Our study provides evidence that C was being lost from the intensive wheat-maize double cropping system in the NCP at a rate of 77gCm-2year-1 when harvest removals were considered, even though crop residue C was inputted into the soil since 30 years ago.