Study On Seasonal Variation Characteristics And Transformation Process Of Soil Nitrogen In Yellow River Estuary Wetland

Yellow River estuary wetland which is located in the south of Bohai Bay and the west of Laizhou Bay, is typical estuarine and coastal wetland ecosystem. In this paper, special distribution pattern and seasonal variations of nitrogen in Yellow River estuary wetland were studied, and the reserves of nitrogen in this field were estimated;mineralization, nitrification and denitrification of nitrogen were measured by culture experiments in the lab. The amount of transformational organic nitrogen during mineralization and transformational ammonia nitrogen during nitrification, and the loss of nitrate nitrogen during denitrification were estimated respectively; the reed decomposition experiment was done in the filed and the amount of reed returned to soil was estimated. Based on laboratorial and field research, biological processes of nitrogen were systematically analyzed, thus the mechanism of the biogeochemical processes in terms of nitrogen and the main elements affecting this process were discussed. And also the potential ecological effects generated by nitrogen removal and transformation in wetlands were also analyzed. This study could provide scientific gist and guidance in theory for sustainable utilization,management and protection of Yellow River estuary wetland.Studies on spatial distribution of nitrogen in Yellow River estuary wetland showed that:nitrogen content in agriculture area soil was higher than that in natural wetlands; organic nitrogen was the main form in soils which account for up to 98% of total N content, as ammonium nitrogen account for about 80% of inorganic nitrogen content. The nitrogen content in the surface soil lied in a low nutrition level, with ammonium nitrogen content less than 10mg/kg, nitrate content less than 3mg/kg, total nitrogen content lower than 1000mg/kg in the natural wetlands. In the vertical direction, nitrogen content decreased with the depth of the soil in sampling HHK-14-HHK-62; the nitrogen content of 0-20cm was significantly higer than that below 20cm, while the soil nitrogen content below 30cm had little change. With the difference of temperature,moisture conditions and vegetation growth, seasonal variation of inorganic nitrogen was much more obvious than organic nitrogen. The main reasons for the different spatial distribution of nitrogen rely on the vegetation distribution; soil texture, soil organic carbon,transmission processes of nitrogen and so on. Soil nitrogen reserves are different in different seansons.Nitrogen reserves in 0-60cm were 2047.77-3540.44g·m-3 in May,1177.18-2682.38g·m-3 in August, 1540.09-2759.78 g·m-3, in October.Studies on soil nitrogen transformation processes showed that:the ability of nitrogen transformation in HHK-14 whose main plant is Tamarix is higher than that in HHK-21 whose main plant is reed. Nitrogen transformation ability is higher in 0-20cm than that below 20cm,and denitrification capacities are higher than mineralization and nitrification in the two sampling sites.During transformational process, the The maximum amount of organic nitrogen transformation, ammonia nitrogen transformation, nitrate nitrogen loss in soils with 60cm depth in the two study sites are separately 0.91g·m-3·d-1 12.77g·m-3·d-1 and 23.44 g·m-3·d-1.The reed decomposition experiment showed that:The N contents decreased gradually during the decomposition. In intermittent flooding environment, the reduced contents of N of reed aboveground, underground stems, and roots underground were respectively 35.96%-53.49%,8.62%-76.77%,31.20%-72.43%; in non-flooded environment, the reduced contents of N of reed aboveground, underground stems, and roots underground were respectively 16.03%-50.34%,14.68%-69.33%,37.65%-73.77%. NAI<100% indicates that nitrogen is net released during the decomposition of reed. The amount of returned N of reed is highest in summer, lowest in winter,with the amount of nitrogen returned of reed underground higher than that of aboveground.In short, nitrogen content is the lower middle level in soil of Yellow River estuary wetland. Soil texture, soil organic carbon and wetland plants are the major factors affecting nitrogen spatial distribution. In general, nitrogen content is high in areas with a high proportion of clay, high organic carbon content and rich vegetation. Nitrogen reserves are closely related to nitrogen transformation.The lower mineralization and higher denitrification are consistent with lower nitrate content in the study area. The returned amount of Nitrogen of reed influences reserves of soil nitrogen.The returned amount of Nitrogen of reed is up to 0.039g·m-2d-1 during decomposition process.