Resumo

Even though anaerobic methane oxidation (AMO) has been described in several aquatic ecosystems, there are no conclusive results showing their role in soils. However, considering the high diversity of microhabitats present in soils one could expect that AMO might occur in these environments. This might be particularly true for overwintering pastures were the following factors are known to steer microbial communities: (i) dung and urea, the main organic inputs, are enriched in these soils leading to extraordinary amounts of organic carbon and nitrogen; (ii) compaction of soil by animal traffic and other changes reduce soil aeration; and (iii) grazing, trampling and defoliation result in reduced plant N uptake. Therefore, we used a soil from an overwintering pasture in South-Bohemia (Czech Republic) to investigate AOM. Very high methane emissions and high abundance of methanogens were detected in this area, providing strong evidence for the presence of anaerobic micro-sites. We followed CO2 production in soil samples incubated with 1000, 2000, 3000 ppm of 13C- CH⁴ under anaerobic conditions. Our results show an increase in 13C0 ² concentration and changes in the isotopic pattern of microbial biomass. Moreover, the increase of the δ 13 C values of the microbial biomass was associated to the CH⁴ concentration, confirming that AOM is actually occurring. We then tried to determinate which electron acceptors might be used in this process. By now only sulfate and nitrite were described as electron acceptors in AOM, although theoretically iron III could also be used. Sulfate was below detection limit and hence assumed be irrelevant for this soil. However, although iron III and nitrate were available, only nitrate concentration decreased during the experiment. Nitrate amendment did not influence 13C0 ² emissions. The system was not nitrate limited, probably due to the high amounts of N derived from animal urine. Moreover, even though phospholipid fatty acids analysis by GC-MS already indicates that Gram-negative bacteria may be involved in the AOM, stable isotope probing of RNA extracts should confirm this finding.