A Comparison Of Methane And Nitrous Oxide Emissions Between Paddy Fields And Crab/Fish Farming Wetlands In Southeast China

Wetlands are important sources of atmospheric methane(CH4)and(N2O).In recent years,more and more agricultural lands have been developed into freshwater aquaculture wetlands with the rapid development of aquaculture in China.In particular,the conventional rice paddies have been increasingly converted to crab and/or fish farming aquaculture wetlands in southeast China.However,few measurements of CH4 and N2O fluxes have been taken in the aquaculture wetlands,which limits our knowledge about their emission intensities and global budget.Therefore,field measurements of CH4 and N2O fluxes from the typical aquaculture wetlands would help to minimize the uncertainty in estimation of greenhouse gases which emissions from aquaculture wetlands worldwide.This study was carried out in the conventional paddy fields(rice-winter wheat rotations),crab and fish farming wetlands,both converted from adjacent paddy fields.CH4and N2O fluxes were simultaneously measured using static opaque chamber-gas chromatograph(GC)method.The main objectives of this study were to gain an insight into CH4 and N2O emission intensity,emission characteristics,driving mechanism,and their net global warming potential(GWP)in rice paddies and crab/fish aquaculture wetlands.The experimental components were as follows:in the two rice-wheat rotations cycles from 2013to 2015,it explored the comprehensive influence on CH4 and N2O emissions because of different nitrogen levels fertilized in the rice paddies(PF-C,PF-L and PF-F),which were equal to the total nitrogen content of the annual routine fertilizer with large-scale cultivation locally and annual fodder fed in the crab and fish farming wetlands,respectively;in the two breeding cycles from 2013 to 2015,CH4 and N2O emissions between region with and without aquatic vegetable present were compared comprehensively in the crab farming wetlands;in the annual breeding cycle,CH4 and N2O emissions in the fish farming wetlands were determined and their emission levels were evaluated in the whole freshwater ecosystems simultaneously.It also integrated the research results about the GHGs emissions of the paddy fields and the crab and fish farming wetlands to comprehensively evaluate the GHGs emissions mitigation potential brought by shift in agricultural production regime and estimated the total annual amount of CH4 and N2O emissions of freshwater aquaculture wetlands in China and the world respectively lastly.The main results of this study are as follows:1.Different levels of nitrogen fertilizer can not alter annual emission patterns of CH4and N2O in the paddy fields.Substantial CH4 emissions from the rice fields were mainly during the rice growing season,while N2O emissions were mostly during the following non-rice season(including following fallow period and wheat season).Increasing amount of nitrogen fertilizer can reduce the annual CH4 emission,but remarkably increase the annual N2O emission and their net GWP did not differ significantly over a 100-year horizon.In the rice growing season,soil water status and temperature affected CH4 emission significantly;while soil temperature,dissolved organic carbon(DOC)and mineral nitrogen(NH4+-N+NO3--N)contents were main driving factors affecting N2O emission.The results of the two years in-situ measure showed that the annual changes of CH4 and N2O emissions and their net GWP in the rice fields were not different significantly.2.The growth of aquatic vegetation,submerged plant,can not alter annual patterns of the CH4 and N2O emissions,but significantly increase annual cumulative CH4 emission and their net GWP-100 in this region of the crab farming wetland(CFW).The amount of CH4from CFW was mainly during the flooding period,accounting for 94-97%of the annual total CH4 emission;the amount of N2O emission during the drainage and flooding period was similar,while mean seasonal N2O flux of the former was 2-3 times of the latter.According to the observing results of GHGs emissions at different stages in these two typical areas,the average emissions of CH4 and N2O in CFW during the drainage,flooding period and the whole year breeding cycle were 0.04 mg m-2 h-1,0.52 mg m-2 h-1,0.37 mg m-2 h-1 and 50.84?g N2O-N m-2 h-1,21.65 'g N20-N m-2 h-1,30.66?g N2O-N m-2 h-1respectively.Both the physicochemical properties of sediment(sediment temperature,DOC and mineral nitrogen,etc.)and water quality parameters of aquatic water(water temperature,DO,COD and mineral nitrogen,etc.)had significant influences on the CH4and N2O emissions.According to the results in situ measurement for two years,annual variation of the CH4 and N2O emissions and their net GWP in the experimental crab farming wetlands were not significant.3.During the whole year breeding cycle,CH4 and N2O emissions had obvious seasonal patterns and presented the similar trend of seasonal variation in the fish farming wetland(FFW).Annual CH4 and N2O flux averaged 0.48 mg m-2 h-1 and 26.03?g N2O-N m-2 h-1,respectively.The spring and summer were the main periods for the CH4 and N2O emissions in the FFW,and their total seasonal emissions accounted for more than 70%of the annual total emissions,respectively.Similar to the crab farming wetland,both the water quality of the aquatic water(water temperature,DO,mineral nitrogen and COD,etc.)and the physicochemical properties of sediment(sediment temperature,mineral nitrogen and DOC,etc.)were the important factors affecting the CH4 and N2O emissions in the FFW.Due to the unique breeding management measures(e.g drainage,desilting,disinfecting and oxygenating artificially)and the good quality of aquatic water,the fish farming wetlands had relatively lower emissions of CH4 and N20 in the whole freshwater ecosystems.4.Shift in agricultural production regime and pattern of agricultural land utilization can achieve a win-win goal to reduce GHGs emissions from the agricultural ecosystem and increase farmers' income.After conversion,current crab farming wetlands and fish farming wetlands can obviously decrease annual CH4 and N2O emissions and their net GWP compared with the former rice paddies,of which reducing effect of N2O emission was more significant.Over a 100-year horizon,their net GWP was PF-F>PF-L>PF-C>CFW>FFW;net economic profit was CFW>FFW>PF-C>PF-L>PF-F;and GWP-Scaled NEEP(annual GWP of net economic profit)was FFW>CFW>PF-C>PF-L>PF-F.5.The conversion factor of nitrogen by feed into N2O-N(CF-N)in the freshwater aquaculture wetlands was 0.35-0.66%,which was similar with the CF-N of exogenous nitrogen in the freshwater ecosystems,and direct emission factor of N2O-N(EF-Y)was0.18-1.64 g kg-1(the unit yield).The present study estimated annual CH4 and N20-N emissions of Chinese freshwater aquaculture wetlands were to be 0.24 Tg and 5.7 Gg,accounting for 3.0%and 6.1%of annual total CH4 and N2O-N emissions of the paddy fields(rice-winter wheat rotations),respectively in 2013.The world's emission of aquiculture wetlands in 2012 was 13 Gg N2O-N,which accounted for 1.9%and 0.2%of the total N2O-N emissions from the global freshwater ecosystems and anthropogenic total.In China,Hubei,Guangdong,Jiangsu,Hunan,Jiangxi,Anhui,Shandong were key provinces of the CH4 and N2O emissions in the freshwater aquaculture wetlands and some Asian countries such as China,India,Japan and Vietnam were main regions of N2O emissions in the world freshwater aquaculture wetlands.Through comprehensive comparison,these shifts in agricultural production regime and pattern of agricultural land utilization can achieve win-win goals to reduce the GHGs emissions in agricultural ecosystem and increase the economic incomes of farmers.