Differences in methanogenic pathways and methanogenic communities in paddy soils under three typical cropping modes

Abstract

Microbial methane (CH4) production varies among different cropping modes,which has important implications for how to reduce CH4 emissions from paddy fields.However,little is known about the values of anaerobically produced δ13CH4,methanogenic pathways,and their dominant communities in different paddy soils.Through anaerobic incubation experiments and the stable carbon isotope with fluoromethane inhibitor method,CH4 production potential (MPP), the relative contribution of acetoclastic methanogenesis(fac),and the abundance and community composition of methanogens in paddy soils were measured under three typical cropping modes (Rice-Wheat,RW;Rice-Fallow,RF;Double-Rice,DR) in China. The results showed that MPP was 30.7 μg CH4 g–1 d–1 in DR soil,57% and 66% higher than that in RW and RF soils,respectively,possibly due to the lower pH and higher abundance of mcrA genes. Moreover,RF soil had the highest produced δ13CH4 value(–43.9‰) and the lowest produced δ13CO2 value(–26.3‰). Based on the carbon isotope fractionations associated with H2/CO2-dependent methanogenesis(1.049–1.062), the values of fac estimated in RF soil(80–98%)were much higher than that in RW(39–60%) and DR(52–75%) soils. It might be supported by that the Methanosarcina (acetoclastic methanogens) were dominant in RF soil while Methanosarcina and Methanobacterium(hydrogenotrophic methanogens) dominated in RW and DR soils. Redundancy analysis revealed that the community structure of methanogens was significantly affected by soil pH,indicating that the differences in methanogenic pathways under the three typical cropping modes might be caused by the changes in community composition driven by soil pH. The findings suggest that soil pH-induced methanogenic abundance and community composition drive paddy MPP and methanogenic pathways, which would provide important insights into the CH4 reduction in paddies.