Abstract
Paddy fields are important ecosystems, as rice is the primary food source for about half of the world’s population. Paddy fields are impacted by nitrogen fertilization and are a major anthropogenic source of methane. Microbial diversity and methane metabolism were investigated in the upper 60 cm of a paddy soil by qPCR, 16S rRNA gene amplicon sequencing and anoxic 13C-CH4 turnover with a suite of electron acceptors. The bacterial community consisted mainly of Acidobacteria, Chloroflexi, Proteobacteria, Planctomycetes, and Actinobacteria. Among archaea, Euryarchaeota and Bathyarchaeota dominated over Thaumarchaeota in the upper 30 cm of the soil. Bathyarchaeota constituted up to 45% of the total archaeal reads in the top 5 cm. In the methanogenic community, Methanosaeta were generally more abundant than the versatile Methanosarcina. The measured maximum methane production rate was 444 nmol gdwh-1, and the maximum rates of nitrate-, nitrite-, and iron-dependent anaerobic oxidation of methane (AOM) were 57 nmol, 55 nmol, and 56 nmol gdwh-1, respectively, at different depths. qPCR revealed a higher abundance of ‘Candidatus Methanoperedens nitroreducens’ than methanotrophic NC10 phylum bacteria at all depths, except at 60 cm. These results demonstrate that there is substantial potential for AOM in fertilized paddy fields, with ‘Candidatus Methanoperedens nitroreducens’ archaea as a potential important contributor.
Highlights
Methane, a significant greenhouse gas, has up to 34 times the global warming potential over 100 years compared to carbon dioxide (Myhre et al, 2013)
The objectives of this study were (i) to characterize the bacterial and archaeal communities in a paddy field soil core by 16S rRNA gene amplicon sequencing with a focus on methane cycle-related organisms; (ii) determine the abundances of total bacteria, total archaea, ‘Candidatus Methanoperedens nitroreducens,’ NC10 phylum bacteria and Bathyarchaeota; and (iii) estimate the anaerobic methane oxidation potential using nitrate, nitrite and iron as electron acceptors at different soil depths
The highest amount of bacterial copies was observed at a depth of 10 cm (5.6 ± 1.4∗109 16S rRNA gene copies per gdw), and the lowest number was observed at a depth of 60 cm (1.4 ± 0.6∗107 16S rRNA gene copies per gdw)
Summary
A significant greenhouse gas, has up to 34 times the global warming potential over 100 years compared to carbon dioxide (Myhre et al, 2013). The land area designated for rice cultivation is predicted to increase even further. Without mitigation measures, this will result in elevated methane emission to the atmosphere. Alteration in microbial community composition in paddy fields have been studied with respect to flooding (Rui et al, 2009), fertilization and straw application (Bao et al, 2016), temperature (Conrad et al, 2009; Noll et al, 2010), rice cultivar and soil type (Conrad et al, 2008), and plant growth stage (Breidenbach and Conrad, 2015). Paddy fields provide a habitat for both aerobic and anaerobic methanotrophs. The flux of methane to the atmosphere is the net result of production and consumption by methanogenic and methanotrophic microorganisms
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