Study On The Characteristics Of N Leaching Losses And Process Control In Paddy Field In The Yellow River Irrigation Region Of Ningxia

It is an important subject for the sustainable agriculture development to improve nitrogen fertilizeruse efficiency and ensure crop high-yield stability, and then reduce nitrogen loss from farmlands andnegative effect on the environment, and need to coordinate the conflict between food safety, resourcesand environment. Due to the high intensive cultivation of paddy rice in the Yellow River Irrigated Areaof Ningxia, it causes even severe non-point source pollution by unreasonable fertilization and irrigationin the farmlands. Nitrogen leaching losses is the main nitrogen loss pathway, however, these researcheson the characteristics of nitrogen leaching losses and mechanisms of process controlling are notconcerned very much in the paddy rice farmland ecosystem. Therefore, it has important scientific andrealistic significance to study on the characteristics of water pollution in paddy rice field and thecontrolling technology of pollutant in the Yellow River Irrigated Area of Ningxia.There were two different kind of experiments conducted from2010to2012. The soil columnsimulation experiment in a laboratory was used to study the characteristics of soil nitrogen movementand leaching in Irrigation Silting Soils with these treatments of conventional fertilization (N₃₀₀),optimize fertilization (N₂₄₀), and twice as much as the nitrogen fertilizer rates (N₆₀₀, N₄₈₀), and theaccumulated leaching process of different form of nitrogen were fitted with the equations. The fieldin-situ experiment was carried out to study the effects of side bar fertilization （low fertilizer rate, middlefertilizer rate and high fertilizer rate）, seeding-box fertilization （middle N rate and high N rate）, andpostponing N application （topdressing by3⁴times） in contrast to conventional fertilization on the riceyield, nitrogen fertilizer use efficiency, nitrogen leaching loss and apparent nitrogen balance. The mainresults are as following:（1） Soil column simulation experiment indicated that NO₃^--N concentrations in leachate withdifferent fertilization treatments were increased firstly and then decreased, the leaching peak arose delaywith nitrogen rates increasing. The accumulated NO₃^--N leaching amounts were68.37,86.22,146.26,and170.68kg/hm²in treatments N₂₄₀, N₃₀₀, N₄₈₀, and N₆₀₀, which accounted for nitrogen fertilizerapplication rates by28.49%,28.74%,30.47%, and28.45%, respectively. NO₃^--N was the predominantnitrogen leaching form, and accumulated NO₃^--N leaching amount Y_tand leaching time t（d） can befitting use equation, which could be described by the logarithmic equation of Y_t=a+blnt in treatmentsN0, N₂₄₀, and N₃₀₀; with nitrogen application rate increasing, the power function equations of lnY_t=a+blnt were found in treatments N₄₈₀and N₆₀₀.NH₄⁺-N leaching loss occurred in the earlier period. The accumulated NH₄⁺-N leaching amountwere18.60,25.07,37.77, and45.96kg/hm²in treatments N₂₄₀, N₃₀₀, N₄₈₀, and N₆₀₀, which accountedfor nitrogen fertilizer application rates by7.75%,8.36%,7.87%, and7.66%, respectively. Theaccumulative NH₄⁺-N leaching amount and leaching time t（d） was submit to the logarithmic equation ofY_t=a+blnt.Total N leaching losses were94.53,128.02,222.06, and268.6kg/hm²in treatments N₂₄₀, N₃₀₀, N480, and N600, which accounted for nitrogen fertilizer application rates by39.38%,42.67%,46.26%,and44.77%, respectively. About33.76%⁴6.26%of fertilizer nitrogen applied in soil was lost byleaching. The accumulative total N leaching amount can be fitting use logarithmic equation of Y_t=a+blnt.NO₃^--N leaching amount accounted for total N was ranged from62.40%to66.87%, and NH₄⁺-N tototal N was ranged from15.45%¹6.55%, about20%of nitrogen leaching by dissolved organicnitrogen form. After the leaching experiment, NO₃^--N residual peak in the0²0cm soil, but NH₄⁺-Nmainly accumulated in20⁴0cm soil.（2） The field in-site experiment showed that rice yields were not significantly decreased withtreatments side bar fertilization, seeding-box fertilization, and postponing N application comparing withtreatment conventional fertilization, however, there were20%⁶0%of nitrogen fertilizer rate reductionand significantly improved nitrogen fertilizer production and use efficiency.Partial factor productivity from applied nitrogen in treatment side bar fertilization of high fertilizerrate （N176kg/hm²ï¼‰ was49.3kg/kg, which was significantly higher than29.8kg/kg in treatmentconventional fertilization. Nitrogen fertilizer use efficiency in treatments seeding-box fertilization ofhigh N rate （N120kg/hm²ï¼‰ and postponing N application of N240/3（topdressing by3times） wasincreased by20.7%and8.0%in contrast to treatment conventional fertilization, respectively.The key control stage of nitrogen leaching in paddy rice field: as nitrogen source, during the first9days after urea fertilizer application is a critical period to prevent nitrogen leaching loss; there alwayshigh risk of leaching loss within30days after transplanting with the side bar fertilization treatment;however, the surface water total N concentration in treatment seeding-box fertilization was significantlylower than that in treatment side bar fertilization, and the risk of loss was lower.The nitrogen leaching mainly occurred before the rice jointing stage in treatment conventionalfertilization, but the leaching peak shift after the tiller-flowering stage in treatments side barfertilization, seeding-box fertilization, and postponing N application. NO₃^--N is the main form ofnitrogen leaching loss, NO₃^--N leaching amount accounted for total N by66.3%in treatmentconventional fertilization. Compared with treatment conventional fertilization, the net total N leachingamount were declined by11.75,30.46, and14.64kg/hm²in treatments side bar fertilization,seeding-box fertilization, and postponing N application, the leaching ratio decreased by44.5%,51.8%and52.5%, respectively. Total N leaching amounts accounted for nitrogen fertilizer application ratewere7.93%,23.7%and6.74%in treatments side bar fertilization of high fertilizer rate （N176kg/hm²ï¼‰seeding-box fertilization of high N rate （N120kg/hm²ï¼‰ and postponing N application of N240/3,respectively. The average total N leaching rates in treatment conventional fertilization was12.8%.（3） Nitrogen loss way in the paddy rice field: After the rice harvest, the soil Nminaccumulation wassignificantly lower than the level before transplanting and evidently leaching downward. The averageapparent loss amount was176.4kg/hm²and was57.7%in treatment conventional fertilization, and theapparent nitrogen loss rate were19.6%～35.1%,30.8%～35.1%, and32.2%～49.0%in treatments sidebar fertilization, seeding-box fertilization, and postponing N application, respectively. Use urea as nitrogen source, the proportion of nitrogen leaching loss amount account for the apparent loss by about20%, use slow/control-released fertilizer as the nitrogen source, the proportion of nitrogen leaching lossamount account for the apparent loss of about50%, and the remaining losses by ammonia volatilizationand denitrification.Considering the rice yield and environmental benefits, these could be scientific process controltechnology for reduction of nitrogen losses in the paddy field with treatments side bar fertilization ofhigh N rate （N176kg/hm²ï¼‰, seeding-box fertilization of high N rate （N120kg/hm²ï¼‰, postponing Napplication of N240/3.