中文摘要：

农田生态系统在陆地生态系统碳循环中起着重要的作用。农作物是农田生态系统中重要的植被,通过光合作用吸收大量的二氧化碳。不同作物的固碳能力不同,种植结构以及气候条件的变化均会导致农作物碳蓄积量发生变化。以水稻、玉米、大豆为研究对象,对比分析了1980年、1985年、1995年、2003年和2007年三江平原主要市（县）农作物碳蓄积量,并对碳密度与5～9月积温、降水量进行相关分析。结果表明：三江平原主要农作物碳蓄积总量呈逐年增长趋势,农作物碳蓄积总量从1980年的2.40×106t增加到2007年的2.38×107t,增加了8.9%;平均碳密度与5～9月积温呈正相关（R=0.321,p〈0.05）,与5～9月降水量呈显著负相关（R=-0.345,p〈0.05）。温度与降水量相比,降水对碳密度的影响更明显。

英文摘要：

Farmland ecosystem plays an important role in the terrestrial ecosystem carbon cycle.As the main vegetation in farmland ecosystems,crops assimilate a large amount of carbon dioxide through photosynthesis.And the carbon fixation in crops would vary with crop species,planting structure and climatic condition.Rice,corn and soybean were taken as the study objects,the carbon fixation of these crops in the main cities and counties of Sanjiang Plain were compared and analyzed for the year 1980,1985,1995,2003 and 2007.And the correlations between carbon density of crops and accumulated temperature,precipitation were also analyzed.The results indicated that the carbon fixation of the main crops in Sanjiang Plain was in an increasing trend,which increased from 2.40×106t in 1980 to 2.38×107t in 2007,a increase rate of 8.9%.Average carbon density of crops were found to be positive.correlated with the accumulated temperature from May to September（R=0.0.321,p0.05）,but negative correlated with the precipitation from May to September（R=-0.345,p0.05）.The influence of precipitation on crops carbon density would be more significant compared with temperature.