| Sediments islet wetland inundated during wetting season become exposed during drying season annually due to seasonal changes of water level.Concentrations and forms of nitrogen in the overlying water and sediments of islet wetland are affected by this inundating and exposing process(drying-rewetting process).In order to investigate changes of nitrogen in different forms,to identified the main factors affecting the translocation and transformation of nitrogen in drying-rewetting areas,and to provide the theoretical basis for the regional protection of the lake,islet wetlands,which were little influenced by human activities,with different elevation(the point on shore,the point of intersection,and point under water)were selected in East Dongting Lake as research areas in this paper.Sediments from these areas were collected during the dry season and temperate season.Correlations,variabilities and distributions of nitrogen in these sediments and characteristics of sediments were analyzed to preliminarily reveal the influence of dry-rewetting process on nitrogen forms in sediment-water interface.Effects of water level on nitrogen form and distribution were analyzed based on indoor simulate experiment.The results showed that in the dry season the average content of total nitrogen of the wetland sediments was 0.920g/kg,and the coefficient of variation was 0.335.The average content of nitrate nitrogen was 1.379mg/kg,the coefficient of variation was 1.036.The average content of ammonia nitrogen was 10.506mg/kg,the coefficient of variation was 0.861.In the temperate season the average content of total nitrogen of the wetland sediments was 1.084g/kg,and the coefficient of variation was 0.230.The average content of nitrate nitrogen was 3.141mg/kg,the coefficient of variation was 1.797.The average content of ammonia nitrogen was 15.397mg/kg,the coefficient of variation was 0.843.The average contents of total nitrogen,nitrate nitrogen and ammonia nitrogen in sediments collected during temperate season are higher than that collected during the dry season.The variation coefficient of nitrate nitrogen in sediment collected during temperate season is significantly higher than that collected during dry season.However,the variation coefficients of total nitrogen and ammonia nitrogen were less than that collected during dry season.Pearson correlation analysis was performed for variations in the dry season and temperate season.Significant positive correlation between moisture content and water content(r=0.296,p<0.05)and significant negative correlation between water content and clay(r=-0.466,p<0.01)were discovered in the dry season.It was shown that the moisture in the sediments was mainly combined with the powder in the dry season.In temperate season,the higher the moisture content of the sediments,it is more likely to cause the loss of clay.The water content during the temperate season was positively correlated with ammonia nitrogen,and was negatively correlated with nitrate nitrogen.Significant correlation between water content and these variations were not found in dry season.This indicated that the effect of moisture content on nitrogen form was different in different season.The total nitrogen in sediment was significantly correlated with nitrate nitrogen in the two seasons(r=0.420,p<0.05;r=0.389,p<0.05),which indicating that nitrate nitrogen was mainly derived from total nitrogen mineralization.During the dry season,the total nitrogen was positively correlated with the clay and powder,and negatively correlated with the sand.During the temperate season,total nitrogen was positively correlated with clay.It was shown that the total nitrogen,especially organic nitrogen,was present mainly in the clay particles.Sediment samples were separated into shore samples and underwater samples.Nonparametric test found that there was a significant difference in water content,ammonia nitrogen and total nitrogen during dry season.However,significant difference was found in water content and nitrate nitrogen during temperate season.Rewetting had different effects on the nitrogen forms in the sediments in different season.Significant difference was shown in ammonia nitrogen and total nitrogen during dry season.However,significant difference was found in nitrate nitrogen during temperate season.Based on the analysis of nitrogen and properties of sediments from islet wetland,the simulation experiment was carried out in the Greenhouse of Chinese Research Academy of Environmental Sciences with the islet wetland sediments to explore the migration and transformation of nitrogen forms in the sediment-water interface with different change rate of water level,which is 8 cm/d,2.6 cm/d,1.3 cm/d,respectively.The overlying water samples and sediment samples were taken once every other day.Temperature,DO,pH,conductivity of overlying water,pH and ORP of the sediment were tested.TN,NO3--N,NH4+-N,NO2--N of overlying water and sediments were analyzed.Orrelation between physical&chemical parameters and nitrogen forms in overlying water and sediments was analyzing respectively.And the conclusions were shown as follow:The biological activity of temperate season is strong,which is helpful to the transformation of nitrogen form.The average value of total nitrogen,nitrate nitrogen,ammonia nitrogen in temperate season is higher than that in dry season.The increase of water level will reduce the coefficient variation of total nitrogen,ammonia nitrogen,narrowing their spatial difference,but increase the variation coefficient of nitrate nitrogen.The concentration of inorganic nitrogen in the first week will rapidly increase after rewetting,and then begin to slow down.In simulation experiment,nitrogen in the overlying water is dominated by nitrate nitrogen,and the proportion of nitrogen forms is NO3--N>NH4+-N>NO2-N.The total nitrogen,nitrate nitrogen and nitrite nitrogen increase gradually with the rise of water level.However,ammonia nitrogen does not change with water level.The nitrogen in the sediments is dominated by organic nitrogen.And the inorganic nitrogen in sediments is mainly dominated by the ammonium nitrogen.The proportion of nitrogen forms is NH4+-N>NO3--N>NO2--N.With the water level reducing,the ammonium nitrogen in the sediments gradually increase,and the nitrate nitrogen decrease gradually.Temperature,pH,DO(temperature,pH,ORP in sediments)have important effect on the migration and transformation of nitrogen forms in overlying water and sediments.In a certain range,the increase in temperature has a strong role in promoting nitrification.Alkaline environment is conducive to the role of ammonia oxidation,and reduce the pH of the environment.DO or ORP affect the aerobic conversion and anaerobic transformation of nitrogen in water.Nitrogen in the sediment is rapidly migrated into overlying water in the form of ammonium nitrogen with rapid ammoxidation at the water level change rate of 8 cm/d.In samples with the water level change rate of 2.6 cm/d,the TDN(dissolved total nitrogen)in the sediments at the beginning of rewetting was released in a large amount into water.And the ammonia nitrogen released in the overlying water is then gradually adsorbed by the sediments due to the consumption of DO in water.In samples with the water level change rate of 1.3 cm/d,nitrate nitrogen was dominated in the overlying water after three weeks of re wetting. |
PDF Full Text Request