Influence Of Suspended Particles On Denitrification In Agricultural Watershed

Human activities increase the nitrogen load in the water,which leads to a series of environmental problems such as water eutrophication and water quality degradation.Denitrification is the main mechanism of water denitrification,which can eliminate the toxic effect of nitrate accumulation on organisms and reduce the risks of teratogenicity,carcinogenesis,and gene mutation caused by excessive nitrite accumulation in the body.It is estimated that about 50%of the total nitrogen input into rivers worldwide can be removed by denitrification.However,the existing research usually assumes that the denitrification in the water body is dominated by sediment while ignoring the importance of suspended particles in the water body.Suspended particle is ubiquitous in natural water bodies,located between overlying water and sediments in water bodies.Suspended particles have an aerobic and anaerobic microenvironment similar to sediment,and it is easier to obtain nitrate.Compared with overlying water,it is a more inclined attachment carrier for microorganisms.Denitrification with the participation of suspended particles is one of the important links in the denitrification process of the water body,and it is also an important place for the coupled nitrification-denitrification process.Agricultural catchment areas are affected by high-intensity human activities such as fertilization,irrigation,and aquaculture,resulting in high content of activated nitrogen and diverse types of suspended particles in water bodies.The active metabolism of organisms in water,the increase of exogenous suspended particles,and the change in the water environment all make the denitrification process very complex.This is very different from natural rivers and lakes.So what is the impact of suspended particles in agricultural catchment areas on water denitrification?Previous studies mainly measured the denitrification rate of the whole water body（overlying water,suspended particles,sediment-water interface,and sediment）.The two-phase interface between overlying water and sediment was carried out.There were few studies on the role of suspended particles in water bodies,and the contribution of suspended particles in water denitrification was not clear.To explore the temporal and spatial changes in denitrification of suspended particles in agricultural catchments,clarify the influence of suspended particles on the denitrification of water,and build a response model between suspended particles and the denitrification rate of water.Therefore,in this study,the typical pond in Xin Zhuang town agricultural catchment area of Changshu city,Jiangsu province was taken as the research object.Through 9 sampling,54 sampling points,and 162 samples’regular monitoring experiments,the denitrification rate（Den）,suspended matter（SPS）,chlorophyll a（Chl.a）,nitrate-nitrogen（NO₃^--N_w）,and water temperature（T_w）were measured,and the physical and chemical properties of water and sediment were observed simultaneously.Through laboratory experiments,the concentration and composition of suspended particles were controlled,the denitrification rate was determined,and the influence of the concentration and composition of suspended particles on the denitrification of the water body was clarified.The response model of denitrification rate between suspended matter and water body was constructed,and the model was parameterized to get the results and verified.Make clear the contribution of suspended matter to the denitrification rate of the water body.The main conclusions are as follows:（1）The denitrification rate of water in agricultural catchment areas shows strong temporal and spatial heterogeneity,in which the concentration of suspended particles is an important factor.The denitrification rate is the highest in summer and the lowest in winter.The average denitrification rate of fish ponds is the highest,followed by crab ponds and waste ponds.The denitrification rate ranged from 34.69 to 148.5μmol N₂·m^-2·h^-1,and the average total denitrification rate was 73.21μmol N₂·m^-2·h^-1。Through regression analysis between denitrification rate and environmental factors（SPS、Chl.a、NH₄⁺-N_w、NO₃^--N_w、T_w、TN、TP、DOC、NH₄⁺-N、DO、p H）,it is found that denitrification rate has a significant correlation with variables（SPS、TN、TP、NH₄⁺-N_w、NO₃^--N_w、T_w）.（2）The correlation between denitrification rate and suspended particle concentration was further confirmed by an indoor control test.This study designed a method that can directly determine the denitrification rate of suspended particles in water,so that it can simply,effectively,conveniently,and accurately determine the contribution of suspended particles to the denitrification rate.Taking the suspended particles with different compositions（algae,sediment,and organic matter）as the research object,the concentration gradient experiment of suspended particles was set to simulate the suspended particles with different concentrations and determine the denitrification rate.The results show that the denitrification rate increases gradually with the increase of suspended particles concentration,and different kinds of suspended particles can mediate the coupling nitrification-denitrification,which is conducive to denitrification and denitrification.Among them,the linear fitting slope of suspended particles dominated by algae is 1.06,which has the greatest impact on the denitrification rate.The linear fitting slope of suspended particles dominated by sediment is slightly lower,which is 1.04,followed by the denitrification rate.The lowest slope is that the suspended particles dominated by the organic matter are 0.62,which has less effect on the denitrification rate than the first two.（3）The estimation model of the denitrification rate of suspended particles in agricultural catchment areas was constructed,and the measured data（54 groups）were used for parameterization（40 groups）and verification（14 groups）:Den=41.857*（NO₃^--N_w）+0.849*（T_w）+34.471（conventional factor model）,Den=1.959*（SPS）+33.908*（NO₃^--N_w）+1.003*（T_w）+7.205（add SPS model）.Increasing the SPS factor can improve the interpretation of the model from 68.1%to 75.1%.To realize the rapid estimation of denitrification consumption in water on a regional scale,based on the remote sensing factor model of denitrification rate in the early stage of the research group（considering chlorophyll and water temperature）,this study further improved and established the remote sensing factor model considering suspended particles factor:Den=-0.019*（Chl.a）²+1.979*（Chl.a）+1.569*（T_w）+7.648（SPS remote sensing model is not added）,Den=-0.014*（Chl.a）²+1.262*（Chl.a）+3.119*（SPS）+1.697*（T_w）-25.701（add SPS remote sensing model）.The interpretation degree of the model obtained by remote sensing factors（Chl.a,T_w）can reach 31.2%.The interpretation degree of the model obtained by remote sensing factors（Chl.a,SPS,T_w）can reach 52.2%.Based on the above two models,the contribution of suspended particles to water denitrification in the agricultural catchment area is 12.1%～19.5%.This study clarified the influence and contribution of suspended particles in agricultural catchment areas on denitrification for the first time.The denitrification rate model of a water body containing suspended particles is constructed,which provides a tool for rapid and accurate estimation of denitrification in the multi-scale water body.