Response Of Methane Oxidation To Elevated Atmospheric CO₂ Concentration In Paddy Soils

Paddy fields are one of the most important sources of methane（CH₄）emissions,accounting for about 30%of global CH₄ emissions in agricultural ecosystems.Microbial-mediated methane oxidation plays an important role in controlling CH₄ emissions from paddy fields.Currently,the atmospheric carbon dioxide（CO₂）concentration has risen sharply from280μL L^-1 before the industrial revolution to 413.93μL L^-1.The rising CO₂ could affect methanotrophs indirectly through biological and abiotic factors,such as soil carbon input and inorganic nitrogen changes.However,the studies on how the process of methane oxidation in paddy fields responds to elevated CO₂ is not sufficient.Moreover,the increase of atmospheric CO₂ is a process of gradual increase over a long period of time,instead of being fixed after increasing to a certain concentration in a short period.The response of ecosystems caused by abrupt increase in CO₂ concentration may be different from that caused by gradual increase under natural conditions.In this study,gradual increase（GI）and abrupt increase（AI）of atmospheric CO₂ concentration were set up based on the atmospheric CO₂ concentration（AC）using an automatic control platform.The CO₂ concentration of GI treatment was increased by40μL L^-1 annually above AC from 2016 until the increase reached 200μL L^-1,and the concentration of AI was directly increased by 200μL L^-1 above AC since 2016.Here,soil slurry incubation,high-throughput sequencing,and quantitative PCR on pmo A genes were used to systematically investigate the methane oxidation potential,and the abundance and community structure of methanotrophs in paddy soils across four growth stages（tilling,jointing,flowering and milky stages）under different CO₂ treatments in year 2019 and 2020.The results of this study can have important implications for predicting the role of methane oxidation in mitigating climate change and reducing greenhouse gas emissions from agricultural ecosystems under future climate change.The main results are as follows:（1）The methane oxidation potential ranged from 89.2 to 269.2 nmol g^-1 h^-1 and from 108.0to 459.3 nmol g^-1 h^-1 in the studied paddy fields under different CO₂ concentrations in 2019 and2020,respectively.The GI and AI treatments significantly increased the methane oxidation potential in paddy soils.Compared with AC treatment,the GI significantly increased the methane oxidation potential in 2019(CK+160μL L^-1)and 2020(CK+200μL L^-1)by 36.4%and 36.7%,respectively,and the AI significantly increased the methane oxidation potential in2019 and 2020(CK+200μL L^-1 for both years)by 32.4%and 43.8%,respectively.There was no significant difference in the effects of GI and AI treatments on methane oxidation potential in paddy soils.Although different CO₂ increasing modes had different effects on the growth of rice plants and soil physiochemical properties to a certain extent,it was not enough to significantly change the effects of gradual increase and abrupt increase on methane oxidation potential.（2）The pmo A gene abundance varied from 4.47×10⁵ to 8.59×10⁶ copies g^-1 and from1.15×10⁵ to 2.37×10⁷ copies g^-1 under different CO₂ concentrations in 2019 and 2020,respectively.Compared with AC treatment,the pmo A gene abundance was significantly increased by 43.2%and 32.5%,respectively,in 2019 and 2020 under GI,and the abundance was increased by 17.7%in 2019 and significantly increased by 62.4%in 2020 under AI.According to the data,there was a significant difference in the pmo A gene abundance between GI and AI in 2019,while there was no significant difference in 2020.Furthermore,the pmo A gene abundance of methanotrophs was found to have a significant positive correlation with the methane oxidation potential.In addition,the variation trends of the abundance and methane oxidation potential were consistent under different CO₂ treatments,which indicated that the elevated CO₂ promoted the growth of methanotrophs and then enhanced the methane oxidation potential for paddy soils.（3）The type I methanotrophs of Methylococcus,Methylosarcina,Methylomonas,Methylobacter,and Methylocaldum,and type II of Methylocystis,and Methylosinus were detected in paddy soils under different CO₂ treatments.Methylocystis was the dominant methanotrophs in the examined paddy field.The results showed that AI significantly changed the diversity of methanotrophs,which significantly increased the number of OTUs and Shannon index.Moreover,the community composition of methanotrophs also significantly changed with the increase of CO₂.The relative abundance of type I methanotrophs increased significantly under elevated CO₂ while there was no significant difference between AI and GI treatments.In contrast,the relative abundance of type II decreased with the increase of CO₂ concentration,and the relative abundance of type II under GI was significantly lower than that under AI.（4）According to the changes of the key soil physiochmical properties under different CO₂treatments,it is suggested that the elevated atmospheric CO₂ may mainly change the CH₄ and O₂ concentrations and the inorganic nitrogen levels in soils by stimulating the growth of rice,enhancing root secretion,tilling number and aerenchyma of rice.Finally,these changes can have important impacts on methane oxidation potential,pmo A gene abundance and community composition of methanotrophs in paddy soils.