Rice roots and methanogenesis in a paddy soil: ferric iron as an alternative electron acceptor in the rooted soil

Abstract

Irrigated rice fields are among the major sources of the greenhouse gas CH₄. To get a better understanding of the controls of CH₄ production we used microcosms planted with rice and studied emission, porewater chemistry and the potential role of ferric iron as a competitive electron acceptor. CH₄ emission from rice microcosms peaked after 60 d when porewater CH₄ concentrations in the unrooted soil also reached their highest values. A rooted soil layer with low CH₄ concentrations and high E_h developed on top of the lower unrooted soil. When measured in vitro, methanogenesis in the rooted upper soil layer started at a very low rate, but increased dramatically after 25–40 d when ferric iron reduction had stopped. If mixtures of the upper soil layer and of lower unrooted soil layer were incubated, the length of the lag phase depended linearly on the proportion of soil from the upper layer. In the lower soil layer, methanogenesis started immediately at high rates. The gross mineralization rate in both soil layers was identical. There was no difference or change in the numbers of culturable methanogens between the two layers, either at the beginning or at the end of the experiment. Hence, the lag phase in methane production by the upper soil layer may have been caused either by competition for substrates or by direct inhibition, but not by population growth. O₂ release from rice roots controls methanogenesis in the rooted upper soil layer either directly or by the oxidation of ferrous iron. Our data suggest that the presence of ferric iron, resulting from the input of oxygen via the roots, results in a shift of electron flow from methanogenesis to ferric iron reduction. This interaction can be assumed to be of major importance for the biogeochemistry of CH₄ in wetland ricefields.