Mechenism Of Mitigation And Characteristics Of N₂O Emissions From Topical Spring Maize Fields In Western Liaoning Province

In China, amounts of nitrogen （N） fertilizer applied in croplands have been increasing to maintainoptimum yield. The use of N fertilizer plays an important role in ensuring food security. However,overuse of N fertilizer is widespread in crop productions, which has caused severe damages toenvironment. Spring maize is one of the most important crops in Northeastern China. To increase soilfertility and sustain high maize yield, overuse of N fertilizer is widespread in the maize fields in thisregion. The overuse of N fertilizer and intensive tillage could stimulate high emissions of greenhousegases from the maize production in Northeastern China. Therefore, it is crucial for sustaining the maizeproduction systems to reduce emissions of greenhouse gases meanwhile maintain the optimum yields byoptimizing farming management practices. In this study, we focus on mitigating emissions ofgreenhouse gases from maize fields in Western Liaoning Province through optimizing farmingmanagement practices. Field studies, soil microbiology experiments and modeling approach have beenadopted to achieve the research objectives. The results and conclusions from our study were descried asfollowing.（1） The field observations indicate that the high N₂O fluxes were mainly induced by applications ofN fertilizer or rainfall events. The means of seasonal cumulative N₂O emissions and soil respirationswere1.14±0.19kg N ha^-1and2019±264kg C ha^-1, respectively, under farmer’s conventionalfertilization practice （FP） during2009to2012. The inter-seasonal coefficient of variation of seasonalcumulative N₂O emissions and soil respirations were17%and13%, respectively. Further investigationsindicate that the N₂O fluxes were positively correlated （P <0.01） with soil mineral N contents and thesoil respirations were exponentially correlated （P <0.01） with soil temperatures and negativelycorrelated with soil moistures （P <0.01）.（2） Field studies were performed to quantify impacts of alternative N fertilization andincorporation of maize straw on emissions of greenhouse gases and maize yields. In comparison withFP, reducing N application rates by20%（OPT） didn’t significantly mitigate N₂O emissions from thetested field, although slightly reduction can be found under OPT. The treatments with slow-releasefertilizer （CRF） or nitrification inhibitor （OPT+DCD） significantly mitigated N₂O emissions from thetested field, while the crop yields remained unchanged. In comparison with FP, the seasonal cumulativeN₂O emissions in2009and2010were significantly reduced by10%and13%, respectively, under theCRF treatment; the seasonal N₂O emissions in2011and2012were significantly reduced by22%and31%, respectively, under the OPT+DCD treatment. The N₂O emissions under the treatment with maizestraw incorporation （OPTS） were highest among all the treatments in this study, indicating that theincorporation of maize straw increased N₂O emissions from the tested field.（3） The seasonal variation of soil respirations was mainly controlled by crop growth, soiltemperature and incorporation of maize straw in this study. In comparison with the treatments withoutincorporation of crop straw, OPTS obviously stimulated the rates of soil respiration during early stage of maize growing seasons. The means of seasonal cumulative soil respirations during the maize growingperiods from2009to2012were1891.2±59.0kg C ha^-1,1939.3±39.6kg C ha^-1,1945.2±46.3kg C ha^-1,and2096.4±156.8kg C ha^-1, respectively, under CK, FP, OPT, and OPTS. The mean of seasonalcumulative soil respirations was2320.8±102.5kg C ha^-1under CRF during the maize growing periodsin2009and2010. For the OPT+DCD treatment, the mean of seasonal cumulative soil respirations was1759.2±78.0kg C ha^-1during the maize growing periods in2011and2012. Our study indicates thatalternative management practices on N application didn’t obviously affect soil respirations.（4） Soil microbiology experiments were performed during the top-dressing periods in2012. Soilnitrifying and denitrifying bacteria were quantified at the field plots under different treatments toinvestigate the mechanisms resulting in different N₂O emissions among the treatments. The results fromthe soil microbiology experiments show that the average copy numbers of AOB amoA, AOA amoA, andnirS were0.1×105(g SDW^-1),0.2×105(g SDW^-1),0.5×107(g SDW^-1) under CK treatment, respectively;and these values were obviously lower than that under all the treatments with N additions. The soilmicrobiology experiments indicate that the addition of N fertilizer can stimulate the growth of AOB,AOA and nirS gene of denitrifying bacteria, and subsequently stimulate the N₂O emissions. The soilnitrifying and denitrifying microorganism under OPT+DCD were lowest among all the treatments withaddition of N fertilizer, indicating that the OPT+DCD treatment can restrict soil nitrifying anddenitrifying microorganism, and thereby mitigate N₂O emissions.（5） The DNDC model was tested against the filed observations from2010to2011. The resultsfrom the model validation indicate that the simulations of maize yields and seasonal cumulative soilrespirations and N₂O emissions are consistent with the observations. The RMSEs between thesimulations and field observations are less than14%. In addition, DNDC can generally capture thetemporal pattern of daily soil respirations and N₂O emissions. These results indicate the reliability ofusing DNDC to assess the impacts of alternative management practices on crop production andemissions of CO2and N₂O from spring maize fields.（6） We utilized the DNDC model to assess the long-term impacts of alternative managementpractices on maize production and emissions of CO2and N₂O. The modeled impacts of alternativefertilization and crop residue management practices can be summarized as follows: a) optimizing Nfertilization （OPT, CRF, and OPT+DCD） can decrease N₂O emissions （8%,13%, and12%, respectively,for OPT, CRF, and OPT+DCD） while maintain maize yields, however, only slightly increase of soilcarbon can be achieved by optimizing N fertilization; b) optimizing N fertilization in combination withcrop straw amendment may significantly increase soil carbon sequestration, and decrease emissions ofgreenhouse gases while maintain maize yields.