华北平原农田生态系统碳过程与环境效应研究

本文总结了25年来针对华北平原小麦-玉米两熟系统,农田的碳循环对气候变化(温度升高)和管理措施(氮肥施入、秸秆还田和耕作方式等)响应机制的研究成果。自2001年起我们在中国科学院栾城农业生态系统试验站建立了3个长期定位碳循环试验:耕作试验、有机循环试验和增温试验,并完善了4种农田碳过程监测方法体系:隔离罐-碱液吸收CO_2法、静态箱-气相色谱法、涡度相关技术和浓度梯度法。量化了华北平原小麦-玉米两熟系统碳输入-输出的平衡,并对华北平原施氮农田土壤碳截留进行了再评价,指出秸秆还田下高水高肥的精细管理农田正在以77 g(C)·m~(-2)·a~(-1)的速度丢失碳;此外长期氮施入虽然显著增加0~100 cm土体的土壤有机碳含量,但同时会造成0~60 cm土体土壤无机碳含量显著降低。我们在对碳过程环境效应的研究中进一步指出:增温和施氮均会降低CH4汇强度,但对土壤呼吸无显著影响,这可能主要是由于试验增温诱发的土壤干旱抵消了土壤温度的部分影响和土壤呼吸对土壤温度升高的适应性造成的。我们对剖面土壤气体的研究表明施氮对剖面CH4和CO_2均无显著影响。进一步将静态箱法和浓度梯度法相结合的研究结果表明0~40cm土层是北方旱地无氮农田土壤CO_2产生和CH4吸收的主要发生层。

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.