Study On The Characteristics Of N Accumulation And Leaching In Different Farmlands In The Yellow River Irrigation Region Of Ningxia

There are many high intensive farmlands in the Yellow River Irrigation Region of Ningxia;therefore, over-use fertilizers result in even serious agricultural non-point pollution. Lots of studiesfocused on N losses by surface runoff from paddy rice fields; however, N leaching losses is one of theimportant ways resulting in agricultural non-point pollution. These investigations on the characteristicsof soil N accumulation and leaching in different agroecosystems in this region are not concerned verymuch. In order to make clear the characteristics of soil N accumulation and N leaching losses indifferent farmlands and its effect on groundwater in the Yellow River Irrigation Region of Ningxia. Twodifferent typical farmlands of greenhouse vegetable fields and paddy-upland rotation fields were chose,and three different field experiments were conducted from2008to2011, respectively. Fields fixedlocation soil sampling experiment with eight points was used to study the characteristics of Naccumulation and leaching in0¹50cm soil depth, and its effect on groundwater under greenhousevegetable fields and paddy-upland rotation condition, respectively. An in-situ plot field experiment withfive fertilizers treatments was conducted in a greenhouse vegetable field, and the soil leachate samplingwith PVC plates was used to investigate N leaching losses from0⁹0cm intact soil. The fieldexperimental design was a randomized complete block with three replications. A short-term plotexperiment was carried out in a paddy-upland rotation field to study the effects of five differentfertilizers treatments on soil Nminaccumulation and apparent N balance. The field experimental designwas also a randomized complete block with three replications. The main results are as following:Observation of N leaching from two different farmlands showed that the average soluble Ncontents in different soil profiles in greenhouse vegetable fields were significantly higher than that inpaddy-upland rotation fields. Generally, the contents of total soluble N （TSN）, soluble organic N （SON）,and NO₃^--N in greenhouse vegetable fields were1.5⁵.6,1.6⁹.8, and1.5³.4times greater than inpaddy-upland rotation fields, therefore, there was rather higher risk of soil N leaching in greenhousevegetable fields.Soil N accumulation in different farmlands indicated that the main forms of soluble Naccumulation in0¹00cm had no significant differences found in different investigated periods underthe same field condition. However, soluble N accumulation had significant differences betweendifferent fields in the same farmland. Accumulation of TSN, NO₃^--N, and SON in different greenhousevegetable fields were1.6⁴.5,1.4⁴.1, and1.9⁶.1times greater than in the paddy-upland fields, andthe main forms of soluble N accumulation in greenhouse vegetable fields were significantly higher thanin paddy-upland fields.Investigation of groundwater depth and N concentrations from two different farmlands showed thatgroundwater depth was ranged from130²30and86²40cm in greenhouse vegetable fields andpaddy-upland fields, respectively. Concentrations of SON and NO₃^--N in groundwater from greenhousevegetable fields were11.85⁴6.12and0.64⁵.89mg/L, and corresponding in paddy-upland fields were0.12⁴.97and0.03¹.00mg/L, respectively. Therefore, both NO₃^--N and SON are the main N pollution forms in groundwater, and N leaching into groundwater in greenhouse vegetable fields easily result inNO₃^--N concentrations>10mg/L.Determination of N leaching losses from the greenhouse tomato-cucumber rotation systemindicated that in contrast to treatments no fertilizers （CK） and manure （M）, there were significant higherTN, NO₃^--N, and SON leached found in treatments CON, OPT and OPT+C/N during differentinvestigated periods. TN, NO₃^--N, and SON leaching losses in treatments CON, OPT, and OPT+C/Nwere56.1¹28.8,41.7¹04.6, and6.7⁴6.1kg/hm²in tomato, cucumber seasons, and fallow periods,respectively; which were significantly higher than TN10.2⁹9.3, NO₃^--N7.7⁷5.4and SON1.0²5.6kg/hm²in treatments CK and M. Compared with treatment CON, amounts of TN, NO₃^--N, and SONleached in treatment OPT were decreased by1.5%³2.6%,1.6%³5.9%, and6.9%⁴1.4%, respectively.Based on treatment OPT, amounts of TN and SON leached in treatment OPT+C/N were decreased by1.3%¹1.0%and5.0%²7.0%, respectively. Hence reducing fertilizer N rate while adding dairy manureregulated soil C/N ratio could be appropriate fertilization practices for reducing soil N leaching.Investigation of the influencing factors on N leaching and dynamic of N leaching in the greenhousevegetable field showed that dynamics of N leached were very similar to leachate producing, and theleaching peaks were always occurred at flood irrigation in summer fallow and furrow irrigation aftercucumber transplanting. There were positive correlations found between N leached and applied fertilizerN rates and irrigation rates. The dominant N leaching form was NO₃^--N （accounted for57.3%⁹2.0%ofTN）, and the secondary was SON （7.8%⁴2.5%）, only about1%of NH+4-N leached.Soil Nminaccumulation and apparent N balance in the greenhouse vegetable field indicated thatfertilization significantly increased NO₃^--N accumulation in soil profiles in contrast to treatment CK,and there was significant positive linear relationship between soil NO₃^--N accumulation amounts in0¹00cm soil and accumulative applied N rates. In the greenhouse tomato-cucumber rotation system, Nleached accounted for13.4%¹9.5%of total apparent N losses. Therefore, reduced fertilizers N rate orregulating soil C/N ratio could also decrease soil N surplus and apparent N losses in contrast toconventional fertilization.Soil Nminaccumulation and apparent N balance in the paddy-upland rotation system showed thatsoil N accumulation in upland was liable to leaching losses in paddy rice season between paddy andupland converting. In the paddy rice season, the residual NO₃^--N in0¹00cm soil depth was declinedfrom9.7⁶1.9kg/hm²to2.7²6.2kg/hm², and there was no significant difference among differentfertilization treatments. In the paddy-upland rotation system, apparent N losses were decreased by46.9%and33.3%in treatments optimal NPK and combination of NPK and manure in contrast totreatment conventional fertilization, respectively.