Characteristics Of Farmland Greenhouse Gas Emissions And Carbon Footprint Under Different Mulching

The large emissions of greenhouse gases（GHGs,including CO₂,CH₄ and N₂O）are the main causes of global warming and a range of environmental problems.In China,CO₂emissions are mainly generated by energy activities and industrial production processes,which account for 90.4%of the total CO₂ emissions.CH₄ and N₂O emissions are mainly derived from agricultural activities,accounting for 56.6%and 73.8%of the total national emissions,respectively.Straw mulching and film mulching,as effective technologies of increasing soil moisture and temperature,have been widely used in the northern region of China.However,the current researches focus on the effects of straw mulching and film mulching on soil water and temperature,and crop yield.There are few researches on the effects of straw mulching and film mulching on GHG emissions in farmland agriculture.In addition,those studies were based on relatively short-term GHG observations,and lacked comprehensive environmental effect analysis.Therefore,it is of great significance for grain production and ecological environment to ensure crop production capacity while effectively reducing GHG emissions from farmland under mulching applications.A field experiment was carried out in Yangling,Shaanxi Province,the South Loess Plateau of China,during 2013 to 2017.The emission fluxes of soil GHGs（CO₂,CH₄ and N₂O）were monitored using a static opaque chamber and gas chromatography method.The first part of the study was to investigate the effects of mulching patterns and irrigation on crop yield and soil GHG emission during summer maize growing seasons in 2014 and 2015,including three mulching patterns of no mulching,half film mulching,and full film mulching with both rainfed and irrigated farming systems.The fluxes and seasonal cumulative emissions of CO₂,CH₄,and N₂O under different treatments were analyzed.And the effects of mulching modes and irrigation on carbon sequestration and emission reduction were evaluated by greenhouse gas intensity（GHGI）.The second part of the study was based on the four consecutive years of winter wheat-summer maize rotation experiment under irrigated condition.The effects of straw mulching and film mulching（including half film mulching,full film mulching,ridge-furrow planting with film mulching over ridges）on crop yield,total GHG emissions and the components of GHG emissions in winter wheat-summer maize cropping system were studied,using the life cycle assessment（LCA）method and carbon footprint（CF）index.The main results of this study are as follows:（1）Film mulching increased soil moisture and soil surface temperature during the summer maize growing seasons,and the crop yield was thus greatly improved.The increase of crop yield under full film mulching was greater than that under half film mulching.That is,full film mulching presented the best yield increase effect among all the studied treatments.Film mulching significantly increased the soil temperature 3⁸℃in the early stage of maize growing seasons（60 days after sowing）under rainfed condition,but such effect was not significant under irrigated condition.The soil moisture contents of film mulching treatments were generally higher than those of no mulching during both growing seasons.Under rainfed condition,half and full film mulching did not significantly increase yield in 2014,but increased by 19.6%and26.8%,respectively,in 2015,as compared with no mulching treatment.Under irrigated condition,the effect of half mulching on improving crop yield was not significant,while full mulching significantly increased crop yield by 14.1%in 2014,and by 55.8%in 2015.（2）Irrigation significantly contributed to CO₂ emissions only in 2015,while the mulching patterns showed no significant effects on CO₂ emissions.Irrigation had no significant effect on CH₄ absorption,and film mulching represented inhibitory effect on CH₄ absorption.Irrigation had no significant effect on total N₂O emissions,and film mulching significantly reduced the seasonal cumulative emissions by reducing the N₂O emission peaks after fertilization.Irrigation and film mulching could significantly reduce GHGI by increasing yields.Under rainfed condition,half mulching and full mulching,as compared with no mulching,significantly decreased GHGI by 60.1%and 61.7%,respectively,in 2015.Under irrigated condition,half mulching and full mulching,as compared with no mulching,significantly decreased GHGI by39.7%and 53.2%in 2014,respectively,and by 22.2%and 67.5%in 2015,respectively.That is,the effect of full mulching on reducing GHGI was better than that of half mulching.Therefore,for summer maize planting agriculture,full film mulching under irrigated condition might ensure high and stable crop productivity and reduce farmland carbon emission intensity.（3）In the second part of the study,the results showed that the interannual average yields of straw mulching,half film mulching,full film mulching,and ridge film mulching were significantly increased by 24.3%,7.0%,15.9%,and 10%,respectively,compared with no mulching treatment.Straw mulching could improve both winter wheat and summer maize yields,while the effect of increasing wheat yield was more significant in the normal year than the wet year.Compared with no mulching,the average wheat yields were significantly increased by 8.3%,10.9%and,12.2%under half mulching,full mulching and,ridge film mulching,respectively.As for summer maize,only full mulching significantly increased the yield by 21.2%.The calculated total GHG emissions using LCA method under different treatments ranked as:full film mulching>ridge film mulching>half film mulching>straw mulching>no mulching.The contributions of agricultural inputs to the total GHG emissions ranked as:nitrogen fertilizers>films>electricity for irrigation>direct N₂O and CH₄emissions>phosphorus fertilizers>seeds>pesticides.GHGs from the production and transportation of nitrogen fertilizers were the largest contributor to the total GHG emissions,accounting for 41.1⁶1.7%of the total emissions under all the studied treatments.With regard to all the film mulching treatments,GHGs caused by the use of films were the second largest contributor to the total GHG emissions followed by nitrogen fertilizers,accounting for 20.6%,34.4%,and 25.7%of the total emissions with half film mulching,full film mulching,and ridge film mulching applications,respectively.While direct N₂O and CH₄ emissions from farmland only accounted for 3.4¹1.4%of total GHG emissions.（4）Compared with no mulching treatment,straw mulching could reduce the carbon footprint of the winter wheat-summer maize cropping system,while film mulching treatments increased it.The CF values of half mulching,full-film mulching and ridge film mulching were significantly higher than that of no mulching,by 16.9%,29.2%,and 21.2%,respectively.The CF value of straw mulching was significantly reduced by 15%,compared with the no mulching.Therefore,in order to balance the contradiction between crop productivity and environmental sustainability,the straw mulching application is recommended in the South Loess Plateau of China.