Tropical rainforest soils: a source of CO!

Carbon monoxide (CO) is an important trace gas in the atmosphere, with its most important sink being the hydroxyl (OH) radical. OH also serves as a sink for methane (CH₄), so CO directly affects the atmospheric concentration of CH₄, making CO an indirect greenhouse gas.

The natural sources and sinks of CO in ecosystems are still poorly constrained. Generally, soil CO fluxes represent a net balance between biological soil CO uptake and abiotic soil and (senescent) plant CO production. Studies largely from temperate and boreal forests indicate that soils serve as a net sink for CO, but to date uncertainty remains about tropical rain forest soils. Here we report the first direct measurements of soil CO fluxes in a tropical rain forest. In addition, we measured nocturnal CO concentrations, from which ecosystem CO fluxes were estimated.

Net soil surface CO fluxes were dominantly positive (∼1 nmol CO m^-2 s^-1) with highest fluxes measured in the dry season in a seasonally inundated valley. Nocturnal CO concentrations showed CO mixing ratios that consistently increased with time and decreased with height, from which ecosystem CO fluxes in the range of 0.3 and 2.3 nmol CO m^-2 s^-1 were estimated. In addition, in the wet season, a greater valley ecosystem CO production was observed in comparison to measured soil valley CO fluxes, suggesting a contribution of the valley stream to overall CO emissions.

Based on our field observations, we expect that tropical rain forest ecosystems are a net source of CO. Extrapolating our first observation-based tropical rain forest soil emission estimate of ∼1 nmol m^-2 s^-1, a global tropical rain forest soil emission of ∼16.0 Tg CO yr^-1 is estimated. Nevertheless, total ecosystem CO emissions might be higher, since valley streams and inundated areas might represent local CO emission hot spots.