关键词: 光合作用, 氮添加, 科尔沁沙地, CO₂排放

Abstract: In this study, transfer characteristics and the spatial and temporal variation of metal ions were investigated by measuring concentration of heavy metals in soil from waterlevel-fluctuating zone (flooded elevation: 145-175 m; control elevation: 175-185 m) at Zigui, Three Gorges Reservoir area in 2009 and 2015. Using the geo-accumulation index, we evaluated the pollution degree of heavy metals at different elevations in two waterflooded years. The results showed that the contents of heavy metals accumulated in soil at different altitudes both in 2009 and 2015 were in order of Fe>Mn>Cu (Zn). With the increased duration of flooding years, more heavy metals accumulated in secondary shrub land than in abandoned farmland. The accumulation of soil heavy metals was more obvious in low altitude area, and the flooded elevation facilitated the accumulation of Cu and Fe, while the control elevation facilitated the accumulation of Zn and Mn. The migration of heavy metals did occur in soil of various vegetations: with the increase of water level fluctuation period, the contents of Cu and Zn at water elevation in secondary shrub land was increased, up to 41.1%, 23.1%, 50% and 9.3%, and 28.6%, 50%, 38.6% and 56.8%, respectively. The contents of Cu and Zn in abandoned farmland decreased, with decreasing ranges of 28.3%-48.5% and 15.5%-29.9%, respectively. Through pollution assessment (I_geo), it was found that, after seven times of water level fluctuation, the soil in secondary shrub land was mainly affected by comprehensive pollution (including Cu, Zn, Fe and Mn) while the abandoned farmland was mainly affected by compound pollution (including Fe and Mn).

Key words: nitrogen addition, photosynthesis, CO₂ emission, Keerqin Sand Land.