Study On The Characteristic Of Nitrogen Release From Lake Sediments

From the1980s to present, with the fast development of industry and agriculture, a large number of nitrogen, phosphorus and organic pollutant have been discharged artificially into lakes. Lakes eutrophication has become a serious environmental problem quickly. The proportion of eutrophic lakes has increased to88.6%in China. Lake is a natural ecological system and plays a major role both in accepting and purifying terrestrial pollution and in the global nitrogen cycling. Research in the nitrification, denitrifying, anaerobic ammonium oxidation processes and inorganic nitrogen vertical distribution rule in sediments will be practical significance to further study on nitrogen pollution remediation and nitrogen cycling in lake ecosystems.In this study, Lake Donghu, Lake Nanhu and breeding ponds surrounding Lake Nanhu in Wuhan City were choosed as research object. The nitration rate in the study area has been determined by using acetylene inhibition technique and static culture in laboratory. At the same time, we also determine the denitrifying rate and anaerobic ammonium oxidation rate by using15N isotope tracer method and static culture in laboratory. The seasonal and spatial change characteristics of nitrification, denitrification and anaerobic ammonium oxidation have been reached through seasonal sampling from the study area. On the base of the above work, the main environmental factors which affect the nitrification, denitrification and anaerobic ammonium oxidation processes have been analyzed. The main results of research are as follows:(1) The nitrification rate of sediment in the Lake Donghu is from180.1to1063.1μmol·m-2·h-1, and its annual average value is605.4μmol·m-2·h-1and in Lake Nanhu ranges from1282.1to4042.9μmol·m-2·h-1, and its annual mean is2141.2μmol·m-2·h-1. For aquaculture ponds, the nitrification rate changes from229.8to983.3μmol·m-2·h-1, and its annual average value is647.7μmol·m-2·h-1.(2) The denitrification rate of sediment in the Lake Donghu is from405.0to2201.3μmol·m-2·h-1,and its annual average value is906.4μmol·m-2·h-1and in Lake Nanhu ranges from1255.1to2903.0μmol·m-2·h-1, and its annual mean is1935.6μmol·m-2·h-1. For the aquaculture ponds, this value changes from335.5to1075.1μmol·m-2·h-1, and its annual average value is696.4μmol·m-2·h-1. For different denitrification ways, the coupling nitration-denitrification of denitrification in Lake Donghu, Lake Nanhu and breeding ponds account for86.3%,78.1%and88.1%respectively, and the uncoupling nitration-denitrification of denitrification account for13.7%,21.9%and11.9%respectively.(3) The anaerobic ammonium oxidation rate of sediment in the Lake Donghu is from50.3to333.8μmol·m-2·h-1, and its annual average value is127.0μmol·m-2·h-1and in Lake Nanhu in Lake Nanhu ranges from105.0to353.2μmol·m-2·h-1, and its annual mean is177.1μmol·m-2·h-1. For aquaculture ponds, this value changes from74.6to123.9μmol·m-2·h-1, and its annual average value is99.8μmol·m-2·h-1. The N2output of total by anaerobic ammonium oxidation processes in Lake Donghu, Lake Nanhu and breeding ponds account for12.1%,8.2%and13.2%respectively.(4) The nitrification, denitrification and anaerobic ammonium oxidation in study area have clearly seasonal and spatial characteristics. The nitrification and denitrification in sediment are strongest in summer, second in autumn, third in winter, and smallest in spring. Compared with nitrification and denitrification, the seasonal characteristic of anaerobic ammonium oxidation is strongest in summer, second in autumn, and smallest in spring and winter. In the same season, nitrification, denitrification and anaerobic ammonium oxidation is higher apparently in sewage outfall than other area, with a clear decrease along the flow or diffusion path.(5) The temperature, nutrients, ammonia concentration in overlying water and organic matter content in sediment are the main environmental impact factors for nitrification, denitrification and anaerobic ammonium oxidation in the study area. And their degrees of influence achieve significance level (p<0.05). In addition, the denitrification and anaerobic ammonium oxidation are restricted by dissolved oxygen, nitrite concentration in overlying water and interstitial water, with their degree of influence achieving significance level (p<0.05).The mian transformation processes of nitrogen in lake ecosystems have been summarized in this paper. Though the research, the nitrogen conversion rates of sediment in the study area have been obtained, and the denitrification contribution in different ways have been distinguished. The seasonal and spatial characteristics of nitrogen conversion have been revealed, and the main environmental impact factors on it also have been defined. Some prospects of the study on nitrogen cycle have been proposed lastly.