Atmospheric methane uptake by tropical montane forest soils and the contribution of organic layers

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH 4 ), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH 4 consumption (6.2 Tg CH 4 year −1 ), limited data are available on CH 4 exchange from tropical montane forests. We present the results of an extensive study on CH 4 exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH 4 sinks, with decreasing annual uptake rates from 5.9 kg CH 4 –C ha −1 year −1 at 1,000 m to 0.6 kg CH 4 –C ha −1 year −1 at 3,000 m. Topography had no effect on soil atmospheric CH 4 uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH 4 uptake rates, mineral nitrogen content of the mineral soil and with CO 2 emissions indicated that the largest CH 4 uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH 4 uptake was N limited instead of inhibited by NH 4 + . Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH 4 budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests.