[Seasonal variation of nitrogen in Xiaojiang backwater area, Three Gorges Reservoir]

Nitrogen is one of the key nutrients for phytoplankton. Seasonal variation of different forms of nitrogen in Xiaojiang backwater area (XBA) in Three Gorges Reservoir from Mar. 2007 to Mar. 2008 was reported. It was found that average concentration of total nitrogen (TN) during the research period was (1553 +/- 43) microg x L(-1). Although remarkable seasonal variability of TN, it showed a general trend of increase of TN during the research period. Dissolved inorganic nitrogen (DIN) was the major form of nitrogen with an average concentration of (1031 +/- 32) micro x L(-1), amounted up to 66.38% in TN. The relatively high level of nitrate in DIN indicated that XBA remained a strong oxidation environment with a good self-purification generally. Mean value of the concentration of dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were (273 +/- 23) microg x L(-1) and (249 +/- 23) microg x L(-1), taking up to 26.48% and 24.15% in TN separately. However, the DON/PON ratio was only 3.63 +/- 0.93, which represented the intensive nitrogen cycling and high turnover rates. The amount of DIN in TN decreased from 80% in Mar. 2007 to 60% in Mar. 2008, which showed a trend that forms of nitrogen had been transferring from inorganic forms to organic forms. Compared to the lower slope of DIN-TN log-log linear model, the relatively high slope of TON-TN log-log linear model indicated the contribution of total organic nitrogen (TON) to TN would become significant with the increase of TN. Meanwhile, the positive correlation between NH4(+) -N/NO3(-) -N and TON/TN indicated the increase of TON in TN might contribute to a reductive environment in XBA. Nevertheless, it also showed a close relationship between ammonia and DON, but whether the ammonification of DON existed or might become the controlling process of nitrogen cycling in XBA still needs further study.