农田土壤有机碳(SOC)库对粮食安全和全球气候变化具有重要影响,因此,开展农田土壤有机碳储量及其动态变化研究在政治经济和生态环境层面具有重要意义.采用农业生物地球化学模型--DNDC对密云水库上游地区农田土壤有机碳储量及其变化进行模拟研究,首先应用当地实测结果进行模型验证,然后根据当地气候条件、土壤性质和现行农业耕作管理特点等建立GIS区域数据库,并在数据库的支持下进行区域模拟和分析.结果表明:2006年密云水库上游地区214 920 hm~2农田土壤(0～25 cm)的总有机碳储量为7 646×10~6 kg,其中位于河北省境内的该地区63.1%的农田储存了全区68.1%的SOC;平均每公顷农田SOC储量为35 576.1 kg,低于全国平均水平;由于化肥和有机肥投入的增加,经过1 a耕种后,2006年该地区农田SOC储量增加142.5×10~6 kg,整个地区及各区县农田土壤碳收支均为正,是大气CO_2的一个汇.情景分析表明,气温升高对该地区农田SOC积累具有显著的负效应;而提高秸秆还田比例、适量施用化肥、增施有机肥、增加灌溉和采取免耕方式等措施均能有效增加土壤有机碳的积累.%Soil organic carbon (SOC) pools in agricultural lands constitutes an important portion of the global carbon pool and has significant impacts on food security and global climate change. North China is one of the largest food producers where the croplands have been undergoing intensive management practices during the past decades. A well process-based biogeochemical model, DNDC (I.e., DeNitrification-DeCompositon), was adopted in the study to quantify SOC storage and dynamics in croplands of the Upper-stream Watershed of Miyun Reservoir in North China. In the study, we further tested DNDC against our datasets measured at four sites with soybean, or corn planted in Yanqing County, Beijing although the model has been validated against numerous observations obtained in other parts of China. Results from our validation tests were encouraging indicating that DNDC was capable of capturing the trends of SOC dynamics in the target region. To support upscaling of the model simulations to the entire region, we built up a GIS database to hold the spatially differentiated data of climate, soil properties, cropping systems, and agricultural management practices at county scale across the domain. By linking DNDC to the GIS database, we accomplished the regional simulations for the 214 920 hm~2 of croplands in the Upperstream Watershed of Miyun Reservoir. Results from the upscaling simulations indicated that (1) the total agricultural SOC storage in the domain was 7 646×10~6 kg (0～25 cm soil layer) with an average 35 576 kg·hm~(-2), which was lower than the national average; (2) the annual increment of SOC in all the croplands in the domain was about 140×10~6 kg in 2006; and (3) the SOC increase was predicted across the target domain. The results implied that the agricultural soils in the region acted as a sink of atmospheric CO_2 under the current climate, soil and management conditions. Alternative scenarios of climate change or farming management were tested with DNDC at regional scale, and the results indicated that (1) increases in temperature would make notably negative impacts on the agricultural SOC sequestration in the studied domain, and however (2) the SOC storage could still increase if higher fractions of above-ground crop residue were incorporated to the soils, more manure was applied, adequate irrigation was applied, or the conventional tillage was converted to no-till.
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