Elevated atmospheric CO_(2) reduces CH_(4)and N_(2)O emissions under two contrasting rice cultivars from a subtropical paddy field in China

Elevated CO_(2)(eCO_(2))and rice cultivars can strongly alter CH_(4)and N_(2)O emissions from paddy fields.However,detailed information on how their interaction affects greenhouse gas fluxes in the field is still lacking.In this study,we investigated CH_(4)and N_(2)O emissions and rice growth under two contrasting rice cultivars(the strongly and weakly responsive cultivars)in response to eCO_(2),200μmol mol^(-1)higher than the ambient CO_(2)(aCO_(2)),in Chinese subtropical rice systems relying on a multi-year in-situ free-air CO_(2)enrichment platform from 2016 to 2018.The results showed that compared to aCO_(2),eCO_(2)increased rice yield by 7%-31%,while it decreased seasonal cumulative CH_(4)and N_(2)O emissions by 10%-59%and 33%-70%,respectively,regardless of rice cultivar.The decrease in CH_(4) emissions under eCO_(2)was possibly ascribed to the lower CH_(4)production potential(MPP)and the higher CH_(4)oxidation potential(MOP)correlated with the higher soil redox potential(Eh)and O_(2)concentration([O_(2)])in the surface soil.The mitigating effect of eCO_(2)on N_(2)O emissions was likely associated with the reduction of soil soluble N content.The strongly responsive cultivars had lower CH_(4)and N_(2)O emissions than the weakly re sponsive cultivars,and the main reason might be that the former induced higher soil Eh and[O_(2)]in the surface soil and had larger plant biomass and greater N uptake.The findings indicated that breeding strongly responsive cultivars with the potential for greater rice production and lower greenhouse gas emissions is an effective agricultural practice to ensure food security and environmental sustainability under future climate change scenarios.

Responses of the methanogenic pathway and fraction of CH4 oxidization in a flooded paddy soil to rice planting

Rice planting(RP)is significant to methane(CH4_(4))emissions from paddy fields,but its effect on the relative contribution of the acetoclastic methanogenesis to total CH4_(4) production(F_(ac))and the fraction of CH4_(4) oxidized(Fox)is poorly understood.To quantify the responses of the F_(ac) and Fox to RP,we investigated CH4_(4) fluxes,CH4_(4) production and oxidation potentials,dissolved CH4_(4) concentrations,and their stable carbon isotopes in a flooded paddy soil.The mcrA and pmoA gene copies were also determined by quantitative polymerase chain reaction(qPCR).Compared with the unplanted soil(control,CK),the seasonal CH4_(4) emissions from the planted soil were significantly enhanced,13.6 times,resulting in large decreases in the CH4_(4) concentrations in the soil solution.This indicated that much more CH4_(4) was released into the atmosphere by the RP than was stored in the soils.Acetoclastic methanogenesis became more important from the tillering stage(TS)to the ripening stage(RS)for the CK,with F_(ac) values increased from 17%-20% to 46%-55%.With RP,the F_(ac) values were enhanced by 10%-20%,and it significantly increased the copy numbers of the mcrA gene at the four rice stages(TS,booting stage(BS),grain-filling stage(GS),and RS).Furthermore,the effect of the RP on the abundance of the mcrA gene was highly concurrent with the effect on the F_(ac) values.At the TS,the Fox values at the soil-water interface were around 50%-75% for the CK,being 15%-20% lower than those of the RP in the rhizosphere.It increased to 65%-100% at the GS,but was reduced by 20%-30% after the RP.These differences might be because the copy numbers of the pmoA gene were significantly raised at the TS while lowered at the GS by the RP.This was further demonstrated by the strong correlations between the effect of the RP on the abundance of the pmoA gene and the effect on the Fox values.These findings suggest that RP markedly impacts on the abundances of the mcrA and pmoA genes,affecting the pathway of CH4_(4) production and the fraction of CH4_(4) oxidization,respectively.

Effects of elevated CO_(2) concentration on CH_(4) and N_(2)O emissions from paddy fields:A meta-analysis

The inevitable rise of atmospheric CO_(2) concentration plays an important role in regulating the carbon(C)and nitrogen(N)cycling in the rice-cropping system.Elucidating the effects of elevated CO_(2) concentration(ECO_(2))on CH_(4) and N_(2)O emissions from paddy fields is essential for evaluating agricultural production in response to global climate change.In this study,we conducted a global meta-analysis to assess the overall effect of ECO_(2) on CH_(4) and N_(2)O emissions from paddy fields,aiming at providing a guideline for sustainable C and N management in paddy fields under future climate conditions.The results showed that,overall,ECO_(2) significantly increased CH_(4) emissions from rice fields by 23%(P<0.05),but reduced N_(2)O emissions by 22%(P<0.05).With a long duration(>10 yr)of ECO_(2) ,ECO_(2) significantly reduced CH_(4) and N_(2)O emissions from paddy fields by 27%and 53%,respectively(P<0.05).Along with the increasing levels of ECO_(2) ,the stimulating effect of ECO_(2) on CH_(4) emissions showed a trend of"weakening firstly and then strengthening",while its effect on N_(2)O emissions changed from stimulation to inhibition.Agronomy managements(e.g.,N application rates,straw incorporations,water regimes,and rice cultivars)affected the effects of ECO_(2) on CH_(4) and N_(2)O emissions from paddy fields.With no or half amount of straw incorporation,ECO_(2) increased CH_(4) emissions by 27% or 49%(P<0.05)from paddy fields,respectively,while non-significant effects on CH_(4) emissions from paddy fields were observed under full straw incorporation.With the increasing amount of straw incorporation,the reductions in N_(2)O emissions from paddy fields were enhanced by ECO_(2).Compared with a continuous flooding regime,intermittent irrigation weakened the promoted effect on CH_(4) emissions but stimulated the inhibited effect on N_(2)O emissions from paddy fields under ECO_(2).Therefore,under the future condition of ECO_(2),it is recommended to adopt the appropriate agricultural management measures,such as combining straw incorporation and intermittent irrigation,and optimizing N application and using rice cultivars of high-yield with lower emissions.In addition,it is necessary to conduct comprehensive studies at multiscale,with multi-factor,and by multi-method to effectively reduce the uncertainty of quantifying the response of CH_(4) and N_(2)O emissions from paddy fields to future ECO_(2) .