Nitrogen fractions and release in the sediments from the shallow lakes in the middle and lower reaches of the Yangtze River area, China

Different nitrogen (N) fractions from 14 sediments from the shallow lakes in the middle and lower reaches of Yangtze River area before and after N release experiments were investigated, and the content of different N fractions, and contribution of different N fractions to the N released from sediments were also studied. Ion-exchangeable form (IEF-N), carbonate form (CF-N), iron-manganese oxide form (IMOF-N) and organic matter-sulfide form (OSF-N) accounted for 2.72 similar to 17.67%, 0.47 similar to 4.43%, 1.1 similar to 3.49% and 31.05 to 71.61% to total N, respectively. The N released was higher than 50% from IEF-N, approximately 35% from OSF-N, 6 and 8% from CF-N and IMOF-N on the average. Approximately 27.32 similar to 70.02% of IEF-N, 10.37 similar to 32.11% of CF-N, 11.37 similar to 33.43% IMOF-N and 2.02 similar to 8.19% OSF-N were released. For the sediments that were slightly polluted, IEF-N was the main N fraction that may be released and its contribution to total N released was more than 63.07%, for the sediments that TN was higher than 3,540.27 mg center dot kg(-1), OSF-N would become the main N fraction that can be released and its contribution to total N released was more than 45%.