Study on Nitrogen Dynamics at the Sediment-Water Interface of Dongting Lake, China

Eutrophication of lakes and reservoirs has become a worldwide environmental problem, and nitrogen (N) has been recognized as one of the key factors responsible for eutrophication. Nitrogen adsorbed on sediments may be released via chemical and biological processes under changing environmental conditions. Spatial distributions of concentrations of ammonia nitrogen (NH4 (+)-N), nitrate nitrogen (NO3 (-)-N) and total nitrogen (TN) were investigated in sediments and overlying water of Dongting Lake, the second largest freshwater lake in China. The concentration of TN in the sediments exhibited strong spatial variation with relatively high values in the eastern part and relatively low values in the southern part of the lake. The TN concentration in the water of different regions of Dongting Lake was affected by the internal load of sediment N. The vertical distribution of TN in sediment cores showed a decreasing trend with an increase in depth. Concentrations of NH4 (+)-N in the sediment cores decreased with the depth increase until 6-8 cm and then increased slowly. However, concentrations of NO3 (-)-N in the sediment cores showed an opposite trend from those of NH4 (+)-N. A kinetic release experiment of NH4 (+)-N showed that the maximum release rate occurred in the first 5 min and the amount of NH4 (+)-N release reached 77.93-86.34 % of the total amount in 0-10 min. The release of NH4 (+)-N in the surface sediments of Dongting Lake fits a first-order kinetics function.