Effect Of Plastic Film Mulching On Greenhouse Gases Emission From A Rice-rapeseed Rotation Cropland And Its Mitigation Measures

As one of the key factors causing global climate change,the greenhouse effect has become one of the hot issues studied by scholars of countries around the world.A large number of studies have shown that farmland ecosystems are an important source of greenhouse gases.Since the mulching film was introduced from abroad in the1970s,it has been widely used after decades of development.After the plastic film is covered,a physical barrier is formed in soils,which directly blocks the vertical evaporation of water and changes the soil moisture,temperature,microorganisms,nutrients,etc.In the southwest region of China,many farmers apply plastic film mulching technology to minimize the adverse impacts of low temperature and drought in early spring.So what impacts will plastic film mulching have on greenhouse gases emissions from farmland under the cultivation practice of paddy-upland rotation（one of the crucial cultivation patterns for rice in southwest China）?In the background of the global warming climate,these are all scientific issues that need urgent solution.A field experiment was conducted in the Key Field Station for Monitoring Eco-environment of Purple Soil of the Ministry of Agriculture of China,located in the farm of Southwest University,Chongqing.Static opaque chamber and gas chromatography method was employed to study the effect of plastic film mulching on greenhouse gas emission characteristics from a rice-rapeseed in situ for three years in this thesis.The purpose of this study is to investigate in depth the emission patterns of greenhouse gases（CH₄,N₂O and CO₂）and their main control factors in paddy fields covered by mulching film（under the paddy-upland rotation mode）,and to determine the GWP caused by the emissions of CH₄,N₂O and CO₂ from farmland soils.Through potting experiment,in which various nitrification inhibitors were added,the effects of plastic film mulching on the emission of CH₄ and N₂O in rice seasons were discussed.The main results obtained are as follows:（1）Effect of plastic film mulching on CH₄ emission fluxEither the mulching treatment or conventional treatment,the emission trend of CH₄ for the past three years was consistent,in which all showed a clear seasonal emission pattern under the three year trial period.In the three years,the flux of methane-enveloped membranes had little difference with that of conventional treatment（p<0.05）except the second year.In addition,the total amount of CH₄emissions under the mulching treatment for the past three years was 195.08kg·hm^-2,which was 3.81kg·hm^-22 higher than that of conventional treatment（p>0.05）;compared with the conventional treatment（p>0.05）,the average flux of CH₄ 0.77 mg·m²h^-1under the mulching treatment was increased by 0.01 mg·m²h^-1,indicating that mulching has no significant effect on both the total amount and the average flux of CH₄ emissions.The average emission flux of CH₄(2.19 and 2.11 mg·m²h^-1;0.165 and0.195 mg·m²h^-1)in both rice and rapeseed seasons of the three years did not show a significant difference between mulching treatment and conventional treatment（p>0.05）,which indicated that the film had no significant effect on the average flux of CH₄ in any growth period in the three-year cycle.（2）Effect of plastic film mulching on N₂O emission fluxThe N₂O emission under the two treatments showed the same emission trend.The maximum emission peaks were concentrated in the rapeseed season.The maximum discharge flux of the membrane treatment in the third year was4833.27μg·m^-2h^-1,which was higher than the first year（p<0.05）and the second year（p>0.05）.In the whole observation period,there was no significant difference in N₂O emission flux between the mulching treatment and conventional treatment（p>0.05）.The total amount of N₂O emissions under the mulching treatment in the three-year rotation cycle was increasing year by year.There was no significant difference between the first year and the second year（p>0.05）,but there did exist a remarkable difference between the second year and the third year（p<0.05）.Being different from the emission law of mulching treatment,the total amount of N₂O emissions under the conventional treatment in the second year,as the highest among the three years.In addition,the maximum average N₂O emissions were mainly distributed in the rapeseed season,in which the average annual emission flux,respectively shown as614.08μg·m^-2h^-1and 482.84μg·m^-2h^-1,reached the maximum in the third year.Compared with the emissions in the rapeseed season of the first year,it increased by166%and 161%（p<0.05）.The average N₂O emission flux 412.46μg·m^-2h^-11 under mulching treatment was not remarkably different form that of the conventional treatment(388.67μg·m^-2h^-1),indicating that plastic film mulching has no significant impact on the average N₂O emissions in the rotaion rapeseed seasons.（3）Effect of plastic film mulching on net CO₂ emission fluxCO₂ net emission peaks occured during the thriving phase of crop growth,while CO₂ absorption was weak at the early stage of the crop growth.The net fixed amount of CO₂ in rice seasons under the mulching treatment showed an increasing trend year by year,i.e.,the third year>the second year>the first year.The net fixed amount in the third year was 30.96 t·hm^-2,16.06 t·hm^-22 higher than the first year（p<0.05）.In addition,the average net CO₂ flux in the rice season under both treatments was almost the highest in this year.The mulching treatment had an obvious impact on the average net CO₂ emissions during the three-year leisure period,but compared with the conventional treatment,it made little difference in the average net emissions,which indicated that the mulching had no significant effect on the average net emission of CO₂ from the rice-rapeseed rotation cycle.（4）Effect of plastic film mulching on cropland CO₂,CH₄,N₂O and global warming potential（GWP）During the three-year observation period,the annual comprehensive GWP of the mulching treatment for the third year was-15.07tCO₂·hm^-2,1.73 times that of the conventional treatment（p>0.05）,but was significantly lower than the first and second year（p<0.05）.Except for the obvious changes in the leisure period in the third year,the GWPs were evenly distributed in each of the other periods at the two treatments,and the comprehensive GWP under the mulching treatment in rice season,leisure period,and rapeseed season had little difference with that of the conventional treatment in the corresponding periods（p>0.05）,indicating that the mulching treatment had no significant effect on the comprehensive greenhouse gas warming potential of the rice-rapeseed cropping system on the 100a times scale.（5）Effect of plastic film mulching on soil physical and chemical properties of croplandCompared with the conventional treatment,the mulching treatment significantly increased soil urease,organic matter and soil temperature by 32.6%,52.1%,65.2%in the first year,3.7%,16.7%,15.4%the second and 0.55%,1.7%,13.5%the third.As it was shown,except the leisure period in the second year,in which the increase of soil urease was not obvious,in any other time the mulching did contribute to increasing the soil urease（p<0.05）.（6）Effect of different nitrification inhibitors on CH₄ and N₂O emissions in rice seasonsNitrification inhibitors except CP under either mulching treatment or conventional treatment had little impact on CH₄ emissions（p>0.05）.Compared with conventional treatment,the addition of nitration inhibitors such as DCD,CP,and DCD+CP under mulching treatment would decrease the amount of CH₄ emissions by1.99,1.52 and 2.27 kg·hm^-2（p>0.05）.Adding DCD under the mulching treatment has remarkably decreased the CH₄ emissions,while the addition of the other two nitrification inhibitors has increased the emissions to varying degrees.Throughout the observation period,N₂O emissions were significantly reduced after adding nitrification inhibitors such as CP,DCD,and DCD+CP.With the exception of an obvious difference in emissions between DCD+CP mulching treatment and conventional treatment（P<0.05）,there was a certain difference,though not obvious,in emissions between the mulching treatment and conventional treatment under the other three adding modes.The average emission flux of N₂O could be shown as control>CP>DCD+CP>DCD and control>CP>DCD>DCD+CP.