Nitrogen Source Identification And Agronomic Control Technology Of Agricultural Non-point Source Pollution In The Chaohu Lake Region

Agricultural non-point source?ANPS?pollution has been identified as a major cause of water quality deterioration,and severely threatening the safety of social-economic production and human life.However,because of the uncertainty of ANPS pollution discharge time and frequency,universalities of discharge areas,the complexity of occurrence mechanism and the difficulty of simulation and control,the effective governance of ANPS pollution becomes to be considerably hard work.Accordingly,how to understand scientifically and control effectively the ANPS pollution has been an important science and application key issues that need to be solved urgently.ANPS pollution has been found to be a major contributor of water-quality degradation to Chaohu Lake,one of fresh water lake with the most serious eutrophication in China,which restricts the sustainable development of economy and society.Therefore,it is necessary and imperative to trace the ANPS pollutant sources and consequently develop appropriate control technologies with optimal performance in order to successfully control the ANPS pollutant in Chaohu Lake region.The research,choosing Chaohu Lake region as the study unit,has not only analyzed the temporal-spatial variation of various forms of nitrogen contents and isotope values in Dianbu river,the typical tributary of Chaohu Lake region,by means of comprehensive field investigations,nitrogen and oxygen isotopes trace technique(?¹⁵N and?¹⁸O),laboratory analysis in combination with computational model,but also quantitatively evaluated the contribution of four potential sources?atmospheric deposition,AD;NO₃^-derived from soil organic matter nitrification,NS;NO₃^-derived from chemical fertilizer nitrification NF;NO₃^-derived from manure and sewage,M&S?by employing the Bayesian model?stable isotope analysis in R,SIAR?,and explored the characteristics of nutrients at the migration process and in interaction with transport medium in the source-ditch-river system as well.Based upon two long-term field runoff plot experiments,the migration characteristics of soil,water,and different forms of nitrogen,phosphorous at the field under the typical cultivation model in Chaohu Lake region have been studied,and the reduction response of N and P runoff losses from farming field,and the yield effects with various agronomic practices such as plant hedgerow,straw returning,contour cultivation,and optimal fertilization etc.are revealed.The main results are as follows:?1?The combination of qualitative analysis of stable isotopes(?¹⁵N and?¹⁸O)with the quantitative calculation of SIAR indicated that soil nitrogen?NS?,chemical fertilizer?NF?and manure and sewage?M&S?were the major NO₃^-sources to the water of Dianbu River.The source apportionment results showed source contributions differed significantly between the dry and flood season.During the flood season,the major contributors of NO₃^-in the up-stream water were NF with mean contributions of30%,and M&S with mean contributions of 28%,whereas M&S and NS were the dominant NO₃^-sources in the mid-downstream,with mean contributions of 36%and27%,respectively.During the dry season,M&S was the dominant NO₃^-sources,with mean contributions of 38%at the upstream and 48%at the mid-downstream.As a whole,the contributions of different pollution sources,were 7%?18%?atmospheric deposition AD?,24%?29%?NS?,18%?30%?NF?and 28%?48%?M&S?.?2?There existed the obvious spatial-temporal variability of different nitrogen fraction content and nitrate N and O isotopic values in the Dianbu River.In the up-stream,the average concentrations of total nitrogen?TN 4.87 mg/L?and nitrate nitrogen?NO₃^--N 2.73 mg/L?water during the flood season were significantly higher than those during the dry season?3.09 mg/L and 1.17 mg/L?respectively,while ammonium nitrogen concentrations?NH₄⁺-N 1.10 mg/L?during the dry season were higher than that?NH₄⁺-N 0.52 mg/L?during the flood season.At middle-lower reaches,TN,NO₃^--N and NH₄⁺-N concentrations during the flood season were 6.62 mg/L,3.23mg/L and 1.57 mg/L,significantly lower than those?TN 10.52 mg/L,NO₃^--N 4.26 mg/L and NH₄⁺-N 3.66 mg/L?during the dry season.Nitrate is the dominant form of inorganic nitrogen,while the ammonium in sewage.The?¹⁵N-NO₃^-values of water during the flood season?average 5.02‰?were lower than those during the dry reason?average 6.38‰?;However,the?¹⁸O-NO₃^-values during the flood season?with a mean value of 9.17‰?were lower than those during the dry season?with a mean value of4.50‰?.?3?Three conservation measures of plant hedgerow?PH?,plant hedgerow with straw mulching?PHS?and contour ridge?CR?could effectively reduce soil and water losses from sloping cropland in the Chaohu Lake region.The annual runoff and soil losses from sloping cropland at the local conventional tillage treatment in the Chaohu Lake region?CK?were 76.55 mm/a and 767.10 kg/(hm^2.a),respectively.Compared with CK,the runoff at treatments of PH,PHS and CR decreased by 24.5%,36.5%and19.7%,and the soil erosion losses decreased by 31.0%,45.6%and 25.4%,respectively,implying significant effects on soil and water conservation,with better soil conservation than reduction of water runoff losses.Three conservation measures of PH,PHS and CR could effectively reduce TN,PN?particulate nitrogen?and NH₄⁺-N runoff losses from sloping cropland in the Chaohu Lake region.The TN concentrations in runoff in CK ranged from 0.73 mg/L to 22.82mg/L,in which the proportions of PN and DTN?dissolved total nitrogen?in TN were almost similar.NO₃^--N was the major form of DTN in surface runoff?accounting for54.0%?63.7%of DTN?,followed by dissolved organic nitrogen?accounting for22.6%?31.3%of DTN?,and the proportion of NH₄⁺-N was very low?accounting for12.2%?18.7%of DTN?.It was found that the three agronomic measures?PH,PHS and CT?significantly reduced the PN concentrations in runoff,increased the concentrations of DTN,NO₃^--N and DON,and had no obvious influences on the TN and NH₄⁺-N concentrations The annual nitrogen runoff losses from sloping cropland in CK were9.35 kg/?hm2.a?,occupying 2.8%in the applied fertilizer N,respectively,and the proportions of PN,DTN,NO₃^--N?NH4+-N and DON were 50.3%,49.7%,28.6%,8.6%and 12.5%,respectively.Compared with CK,PH,PHS and CR declined TN loads by28.3%,40.7%and 21.2%,PN loads by 58.4%,71.1%and 44.5%?P<0.05?,and NH₄⁺-N loads by 32.8%,48.6%and 28.3%?P<0.05?,respectively.PHS was the most effective for reducing nitrogen losses.We found that the reduction of runoff volume and PN concentrations were mainly responsible for the decreased N losses of these three conservation measures.P,and had no obvious influences on the DOP concentrations?P>0.05?.The annual phosphorous runoff loss from sloping cropland in CK was g/?hm².a?,equivalent to0.98%of the applied phosphorous fertilizer,and the proportions of PP,DTP,D-Ortho-P and DOP were 75.0%,25.0%,22.3%and 2.8%,respectively.Compared with CK,PH,PHS and CR decreased TP loads by 38.4%,53.8%and 33.4%?P<0.05?,PP loads by 49.0%,67.6%and 41.0%?P<0.05?,respectively,and DTP,D-Ortho-P and DOP loads were also decreased at different levels.Similar to nitrogen,the reduction of runoff volume and PP concentrations were mainly responsible for the decreased P losses of these three conservation measures.?4?The first 8-10 days after fertilizer application was the critical period for controlling N and P losses from paddy field in the rice growing season in the Chaohu Lake region.The dynamics of N and P concentrations in standing surface water from paddy field for two consecutive years showed that TN,DTN,NH₄⁺-N and TP concentrations reached the peak values after 2 or 4 days of fertilizer application,then declined rapidly,and finally leveled off after 8-10 days.The concentrations of TN,DTN and TP in surface water could be best described by exponential equation(Y=C₀×e^-kt).Straw return combined with plow tillage could effectively decrease the higher concentrations of TN,DTN,NH₄⁺-N and TP in standing surface water during the earlier stage,and thereby lowered the potential risk of N and P loss in paddy field.?5?The N and P runoff losses from paddy-upland in the Chaohu Lake region showed different characteristics during the rice-growing season and the upland-growing season.N and P runoff losses at paddy-upland rotation experiment with different agronomic practices for 7 consecutive growing seasons?3 rice growing seasons,2 wheat growing seasons and 2 oilseed-rape growing seasons?indicated that N and P runoff losses during the rice growing season were much higher than that during the upland-growing season.TN concentrations in runoff from paddy-upland in the conventional tillage condition varied from 0.73 to 15.33 mg/L,and TP concentrations ranged between 0.07 and 0.50 mg/L.Several higher concentrations all occurred in the rice growing season.DTN and PP were the predominant forms of N and P runoff losses.In the rice growing season,the proportion of NH₄⁺-N and NO₃--N in TN runoff losses varied with the duration of interval between runoff and fertilization,as well as the growth stages of rice.In the upland-growing season,NO₃--N was the major form of DTN,and NH₄⁺-N remained at low proportion.The TN and TP runoff losses from paddy-upland in the conventional tillage condition were 3.07?7.29 kg/hm² and0.24?0.38 kg/hm²,occupying 0.9%?2.2%and 0.36%?0.57%in the applied fertilizer N and P,respectively.Although in controlling the risk of N runoff loss remained remarkable uncertainty due to the contingency of the rainfall event and surface topdressings,the optimized fertilization could decrease the runoff loss of P to some extent.Under either plow tillage or no-tillage,straw return could effectively decline the nitrogen loss,and thereby lowered the potential risk of N loss in paddy field.Although straw return under no-tillage was beneficial to the reduction of P runoff loss in the upland-growing season,it remarkably increased the P loss in the rice growing season.For purpose of controlling the agricultural non-point source pollution,plow tillage combined with straw return was a better practice to reduce nitrogen and phosphorus losses via runoff.In general,the combination of qualitative analysis of stable isotopes(?¹⁵N and?¹⁸O)with quantitative calculation of SIAR reliably identified the nitrogen sources to the water of.Nitrogen loss from farmland was the main nitrogen source in river.Thus,the source control of the nutrient loss via runoff from farmland was the key and most effective strategy for the NPS pollution control in the Chaohu Lake region.Plant hedgerow?Hemerocallis?with straw mulching in slope farmland and plow tillage combined with straw return in paddy-upland were proved to be very effective and efficient in reducing N and P runoff losses and could be applied in the Chaohu Lake region and similar other regions.