Effects Of Elevated CO₂ And Plastic Film Mulching On Greenhouse Gas Emissions In Dryland Maize Field

The Loess Plateau is a typical rain-fed farming area in China.The arid area,including the Loess Plateau,accounts for 43%of the total arable land area in China,and dryland farmland is one of the main sources of greenhouse gas emissions.The greenhouse gases emissions of methane(CH₄),nitrous oxide(N₂O)and carbon dioxide(CO₂)are affected by climatic factors including soil temperature,soil water content,as well as the elevated atmospheric CO₂ concentration.In addition,plastic film mulching,as the main hydrothermal management measure of dryland farmland,plastic mulch is bound to affect greenhouse gas emissions.Therefore,under the background of increasingly intense climate change,it is of great significance for sustainable agricultural development and food security to investigate the coupling effects of elevated atmospheric CO₂ concentration and plastic film mulching on greenhouse gas emissions from farmland in the arid areas.A field experiment was conducted in this study,and the open-top air chambers(OTCs)were used to simulate the environment of elevated atmospheric CO₂ concentration.Five treatments were set:(1)Maize without plastic film covering,without OTC,under natural atmosphere environment(marked as UC);(2)Maize without plastic film covering,with OTC,under natural atmosphere environment(marked as CK);(3)Maize without plastic film covering,with OTC,under elevated atmospheric CO₂concentration at 700?mol·mol^-1(marked as EC);(4)Maize with plastic film covering,with OTC,under natural atmospheric environment(marked as PM);(5)Maize with plastic film mulching,with OTC,under elevated CO₂concentration at 700?mol·mol^-1(marked as PMEC).Each treatment was replicated three times(plots).Nitrogen application rate was the conventional level of 225kg·N·hm^-2.The emission fluxes of CH₄and N₂O and the related influencing factors in farmland ecosystem were observed using static obscure-gas chromatography from 2019 to2020.Soil respiration,soil temperature and moisture were measured at the main growth stages of spring maize including pre-sowing(V0),jointing stage(V6),ninth-leaf stage(V9),silking stage(R1),milk stage(R3),dent stage(R5)and physiological maturity stage(R6)in2019.Greenhouse gas emission rate,cumulative emissions,global warming potential(GWP),greenhouse gas intensity(GHGI)and daily changes of soil respiration were used to evaluate the response of greenhouse gas emissions to elevated CO₂and plastic film mulching in dryland maize field.The main research conclusions are as follows:(1)The experimental results of N₂O emission during the growth period of dryland maize field showed that:In 2019,the average emission rates of N₂O under the treatments of CK,EC,PM and PMEC were 1.07,0.92,1.72 and 2.23 g·N·hm^-2·d^-1,respectively,and the cumulative emission rates were 93.26,91.39,167.05 and 202.43 g·N·hm^-2,respectively.In2020,the average emission rates of CK,EC,PM and PMEC were 4.21,5.15,4.81 and 3.51g·N·hm^-2·d^-1,respectively,and the cumulative emission rates were 329.46,401.43,451.20and 338.07 g·N·hm^-2,respectively.Compared with CK,the treatment of plastic film mulching combined with elevated CO₂(PMEC)significantly increased the N₂O emissions in2019,however,has no significant effect was observed in 2020.Plastic film mulching significantly increased the N₂O emissions during the growth period of maize,while the increase of atmospheric CO₂ concentration had no significant effect on the N₂O emissions.The combined effects of plastic film mulching and elevated CO₂ showed significant inter-annual differences,as the result of changes in precipitation and temperature from year to year.(2)The experimental results of CH₄ absorption during the growth period of dryland maize field showed that:In 2019,the average absorption rates of CH₄ during the growth period for CK,EC,PM and PMEC were 5.50,6.04,4.71 and 4.08 g·C·hm^-2·d^-1,respectively,and the cumulative absorption amounts were 629.34,640.57,510.32 and 468.94 g·C·hm^-2,respectively.In 2020,the average absorption rates of CK,EC,PM and PMEC treatment were 4.19,4.43,3.19 and 2.60 g·C·hm^-2·d^-1,respectively,and the cumulative absorption amounts were 577.85,627.24,464.30 and 383.61 g·C·hm^-2,respectively.Within the two years,plastic film mulching significantly reduced the CH₄ uptake during the growth period of maize,while the increase of atmospheric CO₂ concentration had no significant effect on the CH₄ uptake.The combination of plastic film mulching and the increase of atmospheric CO₂ concentration had a significant inhibition effect on the CH₄ uptake.(3)The GHGI analysis results showed:the GHGI of CK,EC,PM and PMEC treatment in 2019 were 1.98,1.51,3.49 and 4.30 kg·Mg^-1,respectively;in 2020,the GHGI of CK,EC,PM and PMEC were 8.25,10.30,9.62 and 8.41 kg·Mg^-1,respectively.Due to the influence of climate conditions,the effects of plastic film mulching on GHGI significantly increased the GHGI in 2019,the influence of plastic film mulching on GHGI showed an increasing trend,but did not reach the statistically significant level in 2020.The increase of atmospheric CO₂ concentration in the two years had no significant effect on GHGI.The interaction between the two factors of elevated atmospheric CO₂concentration and plastic film mulching on GHGI was similar to that of plastic film mulching mode,which significantly increased the GHGI in 2019,but had no significant effect on the GHGI in 2020.(4)The results of soil respiration of maize under the treatments of UC,CK and EC at V0,V6,V9,R1,R3,R5 and R6 stages showed:Compared with UC,EC increased by 43%and 104%at R3 and R5 stage,respectively(P<0.05);compared with CK,soil respiration rate at R3 and R5 stages was increased by 63%and 109%,respectively(P<0.05).Compared with UC,CK treatment had no significant effect on soil respiration during the whole growth period.The changing dynamics of soil temperature with growth period for the three treatments were consistent,also soil moisture changing dynamics with growth period under the three atmospheric conditions were similar.Soil respiration showed an exponential and parabolic correlation with the soil temperature and soil moisture,respectively.In conclusion,the effects of elevated CO₂ on soil respiration varied with growth stage.Soil temperature and soil moisture were the main factors affecting soil respiration,the elevated CO₂ decreased the temperature effect value(Q₁₀),and increased the threshold value of soil respiration to soil moisture.