Soil Nitrate Removal Ability And Greenhouse Gas Emissions From Riparian Wetland With Continuous Nitrogen Input In Northeast China

Nitrate contamination of surface and groundwater systems has become a major environmental problem globally.In the recent past,riparian wetlands have been considered potential systems for water quality.With the increasing anthropogenic and natural loading of NO3 to riparian wetlands may modify CO2,CH4 and N2O fluxes,thereby affecting the balance of greenhouse gases(GHGs).This study examines how NO3-removal and GHG emissions varied among soils collected from riparian wetland receiving runoff rich in nitrogenous fertilizer and also to understand the influence of soil physicochemical factors have on both the removal of NO3-from receiving water and the emission of GHGs to the atmosphere.Specifically,this study was planned to(?)determine spatial variation and processes involved in NO3-removal in intact soil columns collected from sites dominated by different vegetation types in nitrogenous loaded riparian wetland,(?)determine GHG emissions from intact riparian wetland soil columns continuously loaded with NO3-solution,and(?)assess the effect of water level fluctuation and nitrate concentration levels on soil-surface CO2,CH4 and N2O emissions from riparian freshwater marsh wetland.Intact soil columns collected from different sites dominated by different plant species(Calamagrostis angustifolia,Phragmites australis,Carex pseudo-curaica and Carex schnimdtii)within the wetland were set up in controlled laboratory conditions and treated with NO3-enriched water simulating downward surface water percolating through the soil to become groundwater in a natural system.Other intact soil columns were subjected into varying water table levels(5cm above the above the soil surface,-10cm below the soil water surface and-20cm below the soil water surface)and different NO3-concentration levels.Water and gas samples were collected for the analysis of NO3-and NH4 and C02,CH4 and N2O,respectively.This study revealed remarkable reductions in NO3-concentrations from the inlet to the outlet of the soil columns.Almost 98%of the NO3-disappeared as the water flowed down through the soil columns indicating an efficient nitrate removal capacity of the wetland soils.A significant vertical gradient of NO3-removal rates along the soil profiles were evident with the highest rates occurring in the upper layers.This observation was attributed to differences in TOC along the soil profiles.Nitrate removal at 0 and 10cm depths in sites dominated by P.australis and C schnimdtii was significantly higher than in the site dominated by C.angustifolia.This spatial variability in NO3-was attributed to the quality of organic carbon,C/N ratio and soil pH.The soil collected from the site dominated by C.angustifolia was net C02 and N20 sources,whereas the soil from P.australis and C.schnimdtii were net sinks of C02 and N2O,respectively.Soil from the site dominated by C.schnimdtii had the highest climate impact,as it had the highest GWP attributed to higher CH4 emission compared to the other sites.Our study indicates that TOC and NO3-concentration in the soil water has great influence on GHG fluxes.Carbon dioxide and N2O fluxes were accelerated by the availability of higher NO3-concentration in soil water.On the other hand,higher NO3-concentration in soil water favors CH4 oxidation,hence low CH4 production.The finding of this study implies that higher NO3-concentration in wetland soil,caused by human activities could increase N2O and C02 emissions from the soil.This therefore stresses the importance of controls of NO3-leaching in the mitigation of anthropogenic N2O and C02 emissions.There was significant interaction effect between NO3-concentration and water level group on C02,CH4 and N2O fluxes.Water level fluctuation significantly affected CO2 and N2O fluxes but insignificantly affected CH4 fluxes.The overall mean CO2 emission at 5 cm above the soil surface was two and five times lower than that at-10 cm below the soil surface and-20 cm below the water surface,respectively.The mean CO2 fluxes in C.angustifolia was significantly higher when high NO3-concentration was added in the soil columns while in P.australis and C.pseudo-curaica higher C02 fluxes were observed when low NO3-concentration was added.Nitrate concentration levels significantly affected CH4 fluxes with the highest mean CH4 fluxes values measured when low NO3-concentration was added.Stepwise multiple linear regression revealed that C02 fluxes in C.angustifolia was positively correlated with NO3-and TOC(p<0.05)and negatively with NH4+(p<0.05).In P.australis and C.pseudo-curaica,C02 fluxes were positively and negatively correlated with TOC and NH4+,respectively.Our study implies that there could be considerable change in C02 and CH4 fluxes as a response of water level drawdown due to wetland drainage.Addition of high NO3-concentration leads to(?)increase in C02 fluxes in C.angustifolia likely due to nutritional effect,and(?)reduction in C02 fluxes in C.pseudo-curaica and P.australis probable due to inducing carbon-limitation hence limiting soil respiration.