Anaerobic oxidation of methane in grassland soils used for cattle husbandry

Abstract

While the importance of anaerobic methane oxidation has been reported for marine ecosystems, the role of this process in soils is still questionable. Grasslands used as pastures for cattle overwintering show an increase in anaerobic soil micro-sites caused by animal treading and excrement deposition. Therefore, anaerobic potential methane oxidation activity of severely impacted soil from a cattle winter pasture was investigated in an incubation experiment under anaerobic conditions using ¹³C-labelled methane. We were able to detect a high microbial activity utilizing CH₄ as nutrient source shown by the respiration of ¹³CO₂. Measurements of possible terminal electron acceptors for anaerobic oxidation of methane were carried out. Soil sulfate concentrations were too low to explain the oxidation of the amount of methane added, but enough nitrate and iron(III) were detected. However, only nitrate was consumed during the experiment. ¹³C-PLFA analyses clearly showed the utilization of CH₄ as nutrient source mainly by organisms harbouring 16:1&omega;7 PLFAs. These lipids were also found as most ¹³C-enriched fatty acids by Raghoebarsing et al. (2006) after addition of ¹³CH₄ to an enrichment culture coupling denitrification of nitrate to anaerobic oxidation of methane. This might be an indication for anaerobic oxidation of methane by relatives of "<i>Candidatus</i> Methylomirabilis oxyfera" in the investigated grassland soil under the conditions of the incubation experiment.