Research On Greenhouse Gases Emission Factors And Control Techniques For Rainfed Wheat Field In Northwest China

Climate change is a global environmental problem.Agriculture is a major source of nitrous oxide?N₂O?and methane?CH₄?,which are two important greenhouse gases with global warming potentials 298 and 25 times that of carbon dioxide?CO₂?on a 100-year time scale.The N₂O emissions from croplands are strongly controlled by environmental factors,soil edaphic properties and agricultural practices.In China farming system,over use of synthesis N fertilizers and burning crop residues are common practices,and government makes efforts to prohibit straw burning and to return into the field since recent decades.In order to minimize GHG emission from cropland in China,the first priority is use of optimized N fertilizer rates;the second,use of synthetic N fertilizers accompanied with nitrification inhibitors and the third,use of slow release fertilizers.However,straw stimulates the acceleration of N₂O emission in dry cropland,and emission can be increased considerably when straw was applied coupling with N fertilizers.Therefore,this study was investigated to understand the patterns and processes of greenhouse gas?GHG?emissions from rainfed wheat field in Northwest China,to evaluate the influential factors for N₂O and CH₄ emissions in winter wheat – summer fallow mono-cropping system,and to examine the mitigation measures for greenhouse gas intensity in rainfed cropland.Materials and methods: A two-year field study containing long-term and short-term field experiments was conducted from Oct 2013 to Oct 2015 under rainfed wheat field at the experimental center for the Chinese National Soil Fertility and Fertilizer Efficiency Monitoring Base of Loess Soil in Wuquan village,Yangling district,Northwest China.Randomized Complete Block design was utilized with three replications and each plot occupied 30 m2?6 m x 5 m?for long-term experiment and 4 m2?2 m x 2 m?for short-term experiments?without crop?which investigated the influential factors of GHG emissions.Totally there were seven treatments in this long-term study and they were: control?N0?,conventional N(220 kg N ha^-1)?NCon?,conventional N with straw(SN_Con),optimized N(150 kg N ha^-1)application with straw(SN_Opt),optimized N rate of slow release fertilizer with straw?SSRFOpt?,optimized N with early straw return before fertilization(ESN_Opt),and optimized N with straw and 5% of dicyandiamide?DCD?(SN_Opt+DCD).Urea?46% N?was the main source of N fertilizer except slow release fertilizer?26:6:8?,and phosphorus fertilizer?superphosphate?and potassium fertilizer?potassium sulphate?were applied at the rates of 90 kg P2O5 ha^-1 and 47 kg K2 O ha^-1,respectively for all treatments.All fertilizers were applied as basal fertilizer,and winter wheat?Triticum aestivum L.cv.Xiaoyan 22?was grown in lines with the row spacing of 20 cm by the sowing rate of 120 kg ha^-1.For determination of GHG?N₂O,CO₂ and CH₄?emissions,the closed static chamber method was used and air samples were analyzed by using a modified gas chromatograph?GC System;Agilent Technologies 7890 A,USA?and calculated as ?g N₂O-N m^-2 h^-1 or mg CO₂-C m^-2 h^-1 or ?g CH₄-C m^-2 h^-1.For determination of soil WFPS,soil samples were collected at 0-20 cm depth of top soil.Soil samples were also collected at 0^-100 cm depth of soil profile for the determination of soil mineral N?using standard auto-analyzer techniques?and organic carbon?using the Walkley-Black chromic acid wet oxidation method?.Statistical analysis was carried out using Sigmaplot 12.5?Systat Software Inc.,Erkrath,Germany?.Results and discussion: The net greenhouse gas?NGHG?emissions and net greenhouse gas intensity?NGHGI?were 102 kg CO₂-eq ha^-1 and 23 kg CO₂-eq t^-1 grain in N0 treatment,273 kg CO₂-eq ha^-1 and 44 kg CO₂-eq t^-1 grain in NCon treatment,526 kg CO₂-eq ha^-1 and 88 kg CO₂-eq t^-1 grain in SN_Con treatment,403 kg CO₂-eq ha^-1 and 67 kg CO₂-eq t^-1 grain in SN_Opt treatment,251 kg CO₂-eq ha^-1 and 43 kg CO₂-eq t^-1 grain in ESN_Opt treatment,280 kg CO₂-eq ha^-1 and 45 kg CO₂-eq t^-1 grain in SSRFOpt treatment,and 123 kg CO₂-eq ha^-1 and 18 kg CO₂-eq t^-1 grain in SN_Opt+DCD treatment,respectively.With conventional N rates,straw incorporation increased NGHGI by 102% compared to straw removal,but decreased 24% by reducing 32% of N fertilizer as optimized N rate.It was also observed that the straw incorporation performs noticeable increase in N₂O emissions in winter wheat cropping season.Among the optimized N fertilizer rates compared with SN_Opt treatment,ESN_Opt,SSRFOpt and SN_Opt+DCD treatments decreased in NGHGI by approximately 38%,32% and 73%,respectively.Throughout this study,the N₂O emission factors derived from applied N fertilizer were 0.17-0.21% for NCon,0.22-0.47% for SN_Con,0.38-0.65% for SN_Opt,0.16-0.22% for ESN_Opt,0.22-0.59% for SSRFOpt,and 0.03-0.11% for SN_Opt+DCD treatments.The average N₂O emission factor was 0.37±0.03% derived from N fertilizer and it was less than half of IPCC default values for upland corps.Ultimately,time of early straw incorporation before N fertilization had strong negative relationship with N₂O emission by R² of 0.8031 ?p<0.01?and CO₂ emission by R² of 0.8799?p<0.01?,but fair positive correlation with soil CH₄ uptake by R² of 0.7662?p<0.01?in our rainfed wheat field.Additionally,in this study,no significant relationship between N₂O emission and soil WFPS was observed.Moreover,it was found that soil mineral N and organic carbon contents were not influenced significantly by straw and nitrogen management,and the grain yields were statistically not different among the N fertilized treatments.Conclusion: Through this study,according to patterns and processes of N₂O emission,it suggested that the period 30 – 35 days after N fertilizer application is a crucial emission period?CEP?to evaluate the effectiveness of management practices on N₂O emissions in rainfed mono-cropping system since the statistical differences among annual cumulative emissions coincided with the daily N₂O emission rates during CEP.Straw increases N₂O emissions significantly,and reducing rates of N fertilizer decreases N₂O emissions.Coupling effects of straw and nitrogen management is very important factor for planning strategies on mitigation of greenhouse gas emissions.The 25% higher in the amount of rainfall(almost 300 mm in 2013^-14)during cropping season underwent into 1-2 folds increase in N₂O emissions from N fertilized plots.If we can avoid precipitation or irrigation within 10 days after N fertilization,it can be considerably reduce annual cumulative N₂O emissions.The coated slow release fertilizer can slightly reduce N₂O emissions but not significantly.The early straw incorporation before N fertilization in rainfed upland cropping system can be used as an effective agricultural management practice mitigating GHG emissions without significant decrease in grain yield by decreasing N₂O emissions and increasing soil CH₄ uptakes.Use of DCD mixing into applied N fertilizer is the most effective for mitigation of NGHGI by minimizing N₂O emissions with increasing grain yield in loess soil.