The Enzyme Activities And Escherichia Coli Migration In Soil Under Subsurface Drip Irrigation Applying Sewage Effluent

Drip irrigation is a sound way to apply treated sewage effluent for avoiding contact pollution and decreasing the runoff into the surface water.However,there are some undesirable constituents in treated sewage effluent,such as salinity,nutrient,dissolved organic matter and pathogens.The undesirable constituents may accumulate in the root zone to interrupt nutrients conversion,decrease biological activity and increase the risk of nutrient and pathogens leaching into groundwater while using drip irrigation applying sewage effluent.Accordingly,it is quite necessary to investigate the effects of lateral depth and irrigation level on enzyme activities and Escherichia coli?E.coli?distribution in soil as well as nitrate leaching for subsurface drip irrigation applying treated sewage effluent.Field experiments were designed,with three lateral depths at 0 cm?D1?,15 cm?D2?,and 30 cm?D3?beneath the soil surface and three irrigation levels of 70%?I1?,100%?12?,and 130%?I3?of crop evapotranspiration?ETC?,to investigate the effects of lateral depth and irrigation level on the soil enzyme activities,E.coli distribution,deep percolation,NO₃^-N leaching,soil salinity,growth and yield of maize under semi-humid conditions in the North China Plain during the growing seasons of 2014 and 2015.Additionally,three control treatments with lateral depths of 0?C1?,15?C2?,and 30?C3?cm were irrigated by groundwater at 100%ETC?12?in 2014 and 130%ETC?I3?in 2015.During the experiments,the irrigation amount was determined to replace the accumulated water use for 100%ETC treatment?I2?,and other irrigation treatments were scaled accordingly.The distributions of soil enzyme activities,E.coli,nitrogen contents,salinity and soil water were determined during the maize growing season.The plant height,leaf area index?LAI?,dry biomass of leaf and stem as well as grain,plant nitrogen uptake,yield and yield determinants of maize were recorded at the key growing stages.Furthermore,a hydrokinetic model of HYDRUS-2D was established to evaluate the influences of nitrogen application rates and nitrogen proportion at the maize growing stages on nitrate leaching for I1D2 treatment more systematically.The NO₃^-N leaching was simulated under varying nitrogen application rates and nitrogen proportion at the maize growing stages.The main conclusions of this study are as follows:?1?The soil water content in 0-20 cm depths significantly increased as irrigation level increased and lateral depth decreased under subsurface drip irrigation applying sewage effluent.Irrigation level and lateral depth imposed an apparent influence on soil nitrate content.Generally,a greater irrigation level and lateral depth increased the migration depth of nitrate and the risk of nitrate leaching.Sewage effluent irrigation increased the nitrate content and decreased the ammonium content in the soil when compared to groundwater irrigation.A shallow lateral depth resulted in higher amplification of soil electric conductivity?ECb?in topsoil,while amplification of ECb at a greater lateral depth was higher in deep soils.The sewage effluent irrigation obviously increased the soil electric conductivity?ECb?in 0-50 cm soil depths,but did not result in salinization during the 2-years experiments.?2?After 2-years irrigation,the soil enzyme activities presented an obviously layered distribution in the soil profile.The effcet of irrigation level on soil enzyme activities depended on soil depths,growth stages,and enzyme type,while lateral depth imposed a significant influence on enzyme activities in the soil.Generally,a greater lateral depth resulted in noticeably higher enzyme activities in deeper soils,while enzyme activities at a shallow lateral depth were higher in topsoil.A significant correlation between soil physicochemical property and enzyme activities was observed in both years.The correlation analysis among enzyme activities and nitrate content revealed that irrigation and fertigation practices had similar influence on the enzyme involved in C,N,and P cycling in soil,and the character of urease activities in soil N cycling was initially facilitating N mineralization and subsequently converted to facilitate N uptake and immobilization during the maize growing seasons.Compared to activities measured prior to sowing,soil enzyme activities were generally enhanced by drip irrigation with either sewage effluent or groundwater.There were no significant differences in enzyme activities between treatments of sewage effluent and groundwater.Therefore,the aforementioned analysis suggested that using subsurface drip irrigation to apply sewage effluent did not inhibiting enzyme activities in the root zone and reduced soil fertility.?3?During the maize growing seasons,the subsurface drip irrigation applying sewage effluent did not lead to the E.coli accumulating in the soil as well as leaching.Moreover,the most of the deep percolation was produced by intensive rainfalls at the early and later growing stages.Drip irrigation also resulted in deep percolation.Generally,a great irrigation level and lateral depth resulted in a great deep percolation and a high nitrate concentration in soli solution.The increasing lateral depths significantly increased the cumulative nitrate leaching,while irrigation level imposed an insignificant effect on cumulative nitrate leaching in both years.The sewage effluent irrigation significantly increased the average nitrate leaching by 65%and 84%than that resulted from groundwater irrigation in 2014 and 2015,respectively.?4?During the both maize growing seasons,the irrigation level and lateral depth imposed an insignificant influence on plant height,LAI,plant biomass,plant nitrogen uptake,and yield as well as yield componets.Compared to groundwater,sewage effluent irrigation imposed an insignificant influence on plant height,LAI,plant biomass,plant nitrogen uptake,quality and yield as well as yield componets.Furthermore,drip irrigation did not induce E.coli contamination in corn kernel while applying sewage effluent.The drip irrigation with irrigation level at 70%ETC may be conductive to water saving and maintaining production as well as reducing nitrate leaching when applying sewage effluent in semi-humid regions of the North China Plain.?5?HYDRUS-2D could accurately simulate the water,nitrate and ammonium transport and transformations in soil under subsurface drip irrigation applying sewage effluent during the maize growing season.The simulated results revealed the cumulative nitrate leaching increased as nitrogen application rate increased.In conclusion,a placement depth of dripline at 15 cm associated with irrigation level of 70%ETC can be conductive to increasing the enzyme activities in the root zone,avoiding sewage effluent contact contamination and E.coli accumulation in the soil,and reducing nitrate leaching as well as maintaining a high production when irrigating maize in semi-humid regions of the North China Plain.