Carbon processes and environmental effects on agro-ecosystem in the North China Plain

The mechanisms of agro-carbon cycle in climate change (experimental warming) and management practices (nitrogen application, straw returning and tillage patterns, etc.) were summarized based on studies during the last 25 years in winter wheat and summer maize double cropping system in the North China Plain in this paper. Three long-term field experiments of carbon cycle were conducted since 2001, including tillage experiment, organic carbon cycle experiment and experimental warming field. Meanwhile, four methodologies for monitoring system performance of carbon processes were established, including isolation tank-alkali absorption method, static chamber-gas chromatography method, eddy covariance observation system and concentration gradient-based method. The carbon budget for input and output was quantified and carbon sequestration via nitrogen addition reassessed in winter wheat and summer maize rotation cropland in the North China Plain. The net ecosystem exchange of CO₂ was partitioned into gross primary production (GPP) and total ecosystem respiration (TER). Meanwhile, net primary productivity (NPP) and soil respiration (SR) were determined to compute autotrophic and heterotrophic respirations. The net carbon budget was calculated seasonally based on NPP and considering carbon input through crop residues and carbon output through grain harvest. We found that winter-wheat system was a carbon sink of 90 g(C)·m^-2, whereas summer maize system was a carbon source of 167 g(C)·m^-2. Thus the double cropping system behaved as a carbon source of 77 g(C)·m^-2 at annual scale, corresponding to an annual average loss rate of nearly 1% in topsoil organic carbon stock during 2003-2008. Our study provided evidence that carbon was lost in intensive wheat-maize double cropping system in the North China Plain at the rate of 77 g(C)·m^-2·a^-1, when harvest removals were considered even though crop residue carbon was input into the soil since 30 years ago. Meanwhile, we found that although nitrogen application in calcareous soil significantly increased soil organic carbon pool in the 0-100 cm, it decreased soil inorganic carbon accumulation in the 0-60 profile. The results of further studies on environmental effects of carbon showed that warming and nitrogen fertilization significantly decreased CH₄ uptake, but had no significant effect on total cumulative soil CO₂ flux. The lack of significant effects of warming on soil respiration had resulted from:1) warming-induced soil drying offsetting the effects of soil temperature increase on carbon emission; 2) adaption of soil respiration to increased temperature. In the soil profile, it was found that nitrogen application had no significant effect on production and fluxes of CH₄ and CO₂. Based on simultaneous measurements of soil surface emissions (static chamber-based method) and of subsurface flux (concentration gradient-based method), we highlighted that the topsoil (0-40 cm) played a critical role in CO₂ production and CH₄ consumption in unfertilized maize-based farmland in the North China Plain.