干湿循环对三峡支流消落带沉积物中可转化态氮及其形态分布的影响

水体富营养化的形成与沉积物中氮素的"源-汇"关系密切,本研究选取三峡典型支流澎溪河消落带上、中、下这3个水文断面,160 m和170 m两个水位高程,0~20、20~40、40~60、60~80、80~100 cm共5个深度的沉积物样品,通过研究其总可转化态氮(TF-N)与各形态可转化态氮含量及分布特征,旨在揭示周期性水位变化对消落带沉积物氮释放的影响.结果表明,澎溪河消落带沉积物总氮含量在313.02~3 255.53 mg·kg-1之间,空间分布上呈上站位(渠口)中站位(高阳)下站位(双江)的趋势;总可转化态氮含量范围为288.54~1 123.27mg·kg-1,均值为639.40 mg·kg-1,空间分布趋势与总氮一致;TF-N中各形态氮的大小顺序为:OSF-N(有机态和硫化物结合态)IMOF-N(铁锰结合态)CF-N(碳酸盐结合态)IEF-N(离子交换态).沉积物中TF-N主要以OSF-N(50.9%)和IMOF-N(33.3%)形态存在.OSF-N很难释放,不易参与氮循环.IMOF-N受水文条件影响显著,表现为在低水位高程和下采样站位沉积物中含量更低.淹水胁迫、水体富营养化等情况下氧含量较低,相对还原条件下有利于其向水体释放.而TF-N及其形态分布在垂直深度上无显著差异.可见,三峡库区特殊调蓄水制度加速了澎溪河下游、低水位高程消落带沉积物中IMOF-N向水体的释放.

Effect of Drought and Subsequent Re-wetting Cycles on Transferable Nitrogen and Its Form Distribution in the Sediment of Water Level Fluctuating Zone in the Tributary of Three Gorge Reservoir Areas

Nitrogen is the most comment source of eutrophication in freshwater systems. In current study, we investigated the various forms of transferable nitrogen in sediments, which potentially contributed to the nitrogen output into waters. Sediments samples were collected in the water level fluctuating zone of Pengxi River crossing three hydrological sections, e.g. upstream, midstream and downstream and two water level altitudes, 160 m and 170 m, with multiple depths for each site, 0-20, 20-40,40-60,60-80, 80-100 cm. To characterize the response of transformation of nitrogen in sediment of the water level fluctuating zone towards cycles of drought and re-wetting processes, we analyzed the content and distribution of transferable nitrogen (TF-N) and its forms. The result showed that the changing of the amount of total nitrogen followed the pattern of upstream>midstream>downstream, the mean value was in the range of 313.02-3255.53mg ·kg^-1, while the content of total transferable nitrogen was on an average of 639.40mg ·kg^-1 and coincided with the pattern of total nitrogen. In addition, TF-N followed the pattern of OSF-N> IMOF-N> OSF-N> IEF-N. It indicated that both OSF-N and IMOF-N dominated the form of TF-N in sediments by proportions of 50.9% and 33.3%, respectively. Since the transformation rate of OSF-N into dissolved phase was relatively slow, its contribution as a source of nitrogen to eutrophication was limited. We, thus, concluded that IMOF-N was the most important fraction which could be potentially affected by water fluctuation and contributed as dissolved nitrogen into water phase. This study indicated that the manner of manipulating water resource in the Three Gorges Reservoir area has raised the potential risk of transforming IMOF-N from sediment into water phase.