Effects Of Elevated CO₂ Concentration On Carbon Flux And Comprehensive Global Warming Potentials In Paddy Fields

The increasing concentration of greenhouse gases(GHGs)is the major cause of global climate change.Carbon dioxide(CO₂),the main greenhouse gas,has been increasing at an average annual rate of 2.4?mol mol^–1in the last decade and is expected to reach 538-670?mol mol^–1by the end of the 21st century.Terrestrial ecosystems play a pivotal role in the global carbon balance and can offset a substantial proportion of anthropogenic CO₂emissions.As an important component of terrestrial ecosystems,agro-ecosystems are uniquely positioned to maintain the global carbon balance due to their short carbon sequestration cycle,high intensity,and considerable storage capacity.In addition,agro-ecosystems are also a major source of methane(CH₄)and nitrous oxide(N₂O)emissions,accounting for roughly50%and 60%of total anthropogenic CH₄ and N₂O emissions,respectively.Elevated CO₂concentration may inevitably have a direct or indirect impact on the carbon and nitrogen cycles of agro-ecosystems,which in turn has feedback effects on the future climate system.China is the largest rice producer in the world,with an arable land area of approximately 29.7×10⁶ha and rice production of about 209.6×10⁶tons,accounting for 30%of the total global rice production.Therefore,an in-depth investigation of the carbon flux characteristics of paddy fields and the response and feedback mechanisms of GHGs emissions to the increasing CO₂concentration is vital for quantitatively assessing the carbon sequestration potential and greenhouse effect of agro-ecosystems in the context of climate change.In this study,an in-situ field experiment was conducted during the 2018-2021 rice growing seasons.The experiment was designed with two levels of CO₂concentration:ambient CO₂concentration and ambient plus 200?mol mol^–1.Each CO₂concentration level was replicated four times with a total of 8 open-top chambers.The diurnal and seasonal dynamic changes of carbon fluxes in paddy fields were measured by the static chamber method of greenhouse gas analyzer,and the factors affecting carbon flux in paddy fields were analyzed in combination with environmental variables,plant growth,and soil parameters.The proportional relationship between CH₄-C and CO₂-C,and the comprehensive greenhouse gas warming potential(GWP)resulting from the net exchanges of CO₂,CH₄,and N₂O were estimated.The adaptation of the DNDC model to simulate GHGs emissions under elevated CO₂concentration was explored.Meanwhile,the sensitivity differences of climate conditions,soil factors,and farm management practices to the regulatory effects of GHGs emissions from paddy fields were quantified.The main results are as follows:(1)Elevated CO₂concentration did not change the diurnal and seasonal dynamics of carbon fluxes,but enhanced the carbon sequestration capacity of paddy fields.The CO₂fluxes under different CO₂concentrations showed obvious diurnal dynamic characteristics,all showing a single-peaked“U”pattern curve.After sunrise,the absolute value of CO₂flux gradually increased with the enhancement of photosynthetically active radiation(PAR),reached the peak of CO₂uptake around noon;then gradually declined with the decrease of PAR,and the paddy fields acted as CO₂source at night.In addition,elevated CO₂concentration did not change the seasonal dynamic patterns of CO₂exchange(NEE),ecosystem respiration(RE),and gross primary productivity(GPP)in paddy fields.Carbon exchange under different CO₂concentrations mainly occurred during the vigorous growth period,and carbon uptake capacity was also strongest during these periods.During the2018-2021 rice growing seasons,the paddy fields under different CO₂concentrations exhibited CO₂sinks,and elevated CO₂concentration significantly increased the cumulative amount of CO₂(CAC)by 9.83-13.68%.(2)Carbon fluxes are influenced by various factors,among which PAR and air temperature(Ta),leaf area index(LAI)and SPAD are the dominant environmental and biological factors driving the seasonal variation of CO₂fluxes in paddy fields,respectively.Elevated CO₂concentration increased plant height,LAI,and biomass to some extent,while there was no significant effect on SPAD.In the four rice growing seasons,elevated CO₂concentration increased the rice height,LAI,and aboveground biomass(AGB)by an average of 0.61%,2.56%,and 10.28%,respectively.Overall,CO₂fluxes were significantly correlated with PAR,Ta,relative humidity(RH),soil temperature(Ts),and soil water moisture(SWC).Multiple stepwise regression analysis showed that a three-factor composite model consisting of PAR,Ta,RH,Ts,and SWC explained 54.8-57.4% of the variation in CO₂fluxes during the 2018-2021 rice growing seasons.Path analysis indicated that PAR and Ta had direct effects on seasonal variation of CO₂fluxes in paddy fields with path coefficients of?0.49 and?0.24,respectively.Moreover,the CAC was significantly correlated with AGB,LAI,and SPAD,where AGB could explain 43-78%of the variation in CAC during the 2018-2021 rice growing seasons.(3)Elevated CO₂concentration promoted CH₄and N₂O emissions,which partially offsets the carbon sink effect of paddy fields,thereby increasing the net global warming potential(NGWP)and greenhouse gas intensity(GHGI).Elevated CO₂concentration increased grain number,thousand grain weight,and percentage of unfilled grain to varying degrees.Overall,elevated CO₂concentration increased rice yield by an average of 11.57%over the four rice growing seasons.The cumulative amount of CH₄and N₂O varied considerably among rice growing seasons due to interannual differences in meteorological conditions,plant growth,and water management.Elevated CO₂concentration increased the cumulative amount of CH₄and N₂O emissions from paddy fields by an average of 43.81%and 8.23%,respectively.Meanwhile,the ratio of CH₄-C to CO₂-C increased by 27.21% over the four rice growing seasons.This suggests that although elevated CO₂concentration increased carbon uptake,it also promoted carbon emission to the atmosphere in the form of CH₄from paddy fields.There were significant differences in the NGWP and GHGI of paddy fields between different CO₂treatments and years,while there was no interactive effect of treatment and year.In the four rice growing seasons,elevated CO₂concentration increased NGWP and GHGI by an average of 131.26%and 131.72%,respectively.(4)The DNDC model with localized parameter adjustment can be applied to simulate carbon and nitrogen cycle studies in paddy fields under elevated CO₂concentrations.In general,the DNDC model could well simulate the seasonal dynamic patterns of soil temperature,CH₄,and N₂O from paddy fields under different CO₂concentrations,and the correlation between the simulated and observed values reached a significant level.However,the simulated values of CH₄and N₂O emissions in paddy fields were relatively higher than the measured observed.Sensitivity analysis results indicated that air temperature,soil bulk density,clay content,and soil organic carbon(SOC)content had significant effects on CH₄emissions under the elevated CO₂concentration;while mid-season baking time,SOC,soil bulk density,and soil p H had significant effects on N₂O emissions from paddy fields.