Nitrogen Distribution and Potential Mobility in Sediments of Three Typical Shallow Urban Lakes in China

Long-term behavior of sediment bound nitrogen (N) in promoting eutrophication of freshwater lakes can be more efficiently evaluated on the basis of different N-forms rather than on the total nitrogen (TN) content alone, these different N-forms include N in ion-extractable form (IEF-N), carbonate form (CF-N), iron-manganese oxides form (IMOF-N), and N bound in organic matter-sulfide (OSF-N). To assess the N contamination in sediment and its risk to aquatic systems, content and vertical distributions of TN and extractable nitrogen (Ex-N) in sediments from three typical shallow urban lakes in China were studied. The study examines the geochemical relationships between N-forms and physicochemical characteristics in the sediments, in addition to N-forms and its content proportion in sediments and the concentrations of total dissolved nitrogen (TDN) in the lake water column. Results show that the TN content decreased with sediment depth (0-50 cm) in the three lakes. The Ex-N percentages of the TN varied in the ranges of 58.8-63.8, 52.0-95.0, and 41.6-55.7% in sediments sampled from Mochou Lake, Xuanwu Lake, and Daming Lake, respectively. Distributions of the N-forms in the sediments showed significant differences between the three lakes. OSF-N and/or IMOF-N were/was the dominant form(s), and IEF-N, CF-N, and the OSF-N in surface sediment were positively correlated to TDN in lake water (increasing concentrations), whereas the IMOF-N in the sediment represented a negative correlation with TDN in lake water (decreasing concentrations). This suggests that the IEF-N, CF-N and OSF-N forms of nitrogen in the surface sediments could be released into the overlying water column of the lake under certain chemical conditions posing a serious threat to the lake water quality. These results can be beneficial for understanding the behavior of nitrogen transferring process between sediment and overlying water of lakes, and provide critical data for lake restoration.