Evidence of aerobic and anaerobic methane oxidation coupled to denitrification in agricultural soils

Agricultural soils may act as either a source or a sink for atmospheric methane (CH4) depending on soil type, aeration, water regimes, nutrient availability and environmental variables. The interaction between CH4 and nitrogen (N) has been identified as one of the major gaps in the global carbon (C) and N cycles. Methane is being considered as a low-cost electron donor for coexisting denitrifiers and the denitrification process may be coupled to either aerobic CH4 oxidation involving direct nitrate/nitrite reduction (partial denitrification), or anaerobic relating predominantly to nitrite/nitric oxide reduction (complete denitrification). It is evidenced from isotopic studies that CH4 production and oxidation could take place simultaneously in agricultural soils at water content above field capacity, linking to the presence of anaerobic microsites and aerobic-anaerobic interface. This results in either aerobic or anaerobic CH4 oxidation coupled to the highest N2O emissions, demonstrating a close relationship between CH4 oxidation and denitrification (partial) processes. Besides the involvement of a microbial consortium in the interactive process, recent advancement with microbiological techniques prove the occurrence of the coupled process by combining aerobic methanotrophs and denitrifiers, as well as oxidization of ammonium and metabolic by-products, releasing N2O as a terminal product. However, the apparent anaerobic phenomenon lacks known genes for dinitrogen (N2) production, but subsequent isotopic labelling reveals that methanotrophs could bypass the denitrification intermediate N2O to produce N2 and oxygen that oxidizes CH4. Further investigations using both advanced molecular microbiology and isotope tracing techniques are necessary to elucidate the nature of the processes, better understand the mechanisms in agricultural soils and develop biotechnological solutions to the issues concerning particularly to climate change.