Observation Of Net Global Warming Potential Under Different Nitrogen Managements In Annual Rice-wheat Rotation Systems

Agroecosystems play a vital role in the global carbon balance and act as a main source of greenhouse gases?GHGs?emissons.Excessive use of chemical nitrogen?N?fertilizer led to a series of environmental problems,including the increasing of GHGs emissions,reducing soil organic carbon contents?SOC?and so on.In order to decrease the amount of chemical N fertilizer application and alleviate the environmental pollution caused by chemical N use in agriculture ecosystems,we proposed new approaches that replace some or all of the chemical N fertilizer with manure.These approaches either could alleviate the agricultural non-point pollutions,or they would have a direct impact on the agriculture ecosystem carbon balance and GHGs emissions.Therefore,this study mean to explore the effects of different N managements on the net greenhouse effect in farmland ecosystem,and to provide a scientific basis for the comprehensive evaluation of crop production,soil carbon sequestration potential and the GHGs emissions.The flooded rice and drained wheat rotation,as one of the major cropping systems in Southeast China,was selected in the present study to simultaneously measure CH₄,N₂O and CO₂ fluxes with a static,opaque chamber-gas chromatograph method?2011-2014?.The primary objective of this study was to gain an insight into the effect of different fertilizer managements on GHGs emissions,?SOC,net ecosystem carbon buget?NECB?and net global warming potential?GWP?.In this study,six treatments were arranged as:control?CK?,N fertilizer?CF?,composted pig manure + N fertilizer?MC?,pig manure organic and inorganic compound fertilizer?CC?,straw + N fertilizer?SC?and straw +straw-decomposing inoculant + N fertilizer?SB?.Also,we want to know the GHG emissions and the carbon loss during the composting process of pig manure,and the GHGs emissions affected by field managements and chemical N fertilizer inputs.The main results of this study are displayed as follows:1.Annual CH₄ cumulative emissions for each fertilizer patterns ranged from 71.60 kg C ha^-1 yr^-1?CK?to 116.84 kg C ha^-1 yr^-1?MC?.Different fertilizer patterns were not significantly affected the CH₄ emissions.Annual N₂O cumulative emissions for each cultivation pattern ranged from 1.42 kg N ha^-1 yr^-1?CK?to 3.05 kg N ha^-1 yr^-1?MC?.Compared with CF treatment,MC treatment significantly increased the N₂O emission,which was not significantly changed in CC treatment.The NECB of each treatment was expressed as carbon sink(1.01 t C ha^-1 yr^-1-1.29 t C ha^-1 yr^-1).Compared with CF treatment,MC and CC treatments significantly increased the carbon sequestration in this rice-wheat rotation ecosystem.Compared to CK treatment,all treatments increased the net GWP and GHGI significantly.Moreover,compared to CF treatment,MC treatment increased the net GWP by 351.7%,while CC treatment reduced net GWP by 3.4%.The GHGIs of all treatments showed consistent results with the net GWPs of them.2.The pattens of chemical N fertilizer reduction with straw returning to field influenced the GHGs emission and carbon sequestration in paddy soils.Straw returning significantly increased CH₄ and N₂O emissions,increased soil carbon sequestration and crop yields.However,the mitigation of greenhouse effect due to carbon sequestration does not offset its direct contribution to GHGs emissions.Compared with CF treatment,two straw returning treatments,namely,SC and SB treatments were increased net GWP by 1479.3%and 1196.6%,and GHGI increased by 1200.0%and 966.7%,respectively.Therefore,the pattens of chemical N fertilizer reduction with straw returning need further researches to achieve the goal of both reduce emissions and increase carbon sequestration.3.The CH₄ and CO₂ emissions rates were high in the early compost stage,while the N₂O emission rate was low in the early compost stage and then increased in the later stage.Water content of the compost was the main factor affecting GHG emissions from the composting process.The GWPs of all treatments had no statistical differencess.During the composting process,organic carbon contents of the four treatments were overall trend to downward,and carbon losses of the four treatments were ranging from 19.9%to 22.6%.The three treatments?MN,MS1 and MS2?all reduced the loss of gaseous form of carbon during the composting process,and the least of carbon loss was observed in MN treatment.4.The GWP from agricultural practices and chemical inputs could not be ignored.The carbon equivalent emissions from fertilizer inputs were much more than those from the agricultural farming operations.Through a comprehensive evaluation of net GWPs and GHGIs for different N managements in typical rice-wheat rotation ecosystem,we found that soil carbon sequestration and carbon emission from fertilizer inputs were important contributions to the GWP.Based on the results of GWP with life-cycle assessment and crop yields,organic and inorganic pig manure compound fertilizer is recommended as the best fertilization strategy.