Long-term CO2 production from deeply weathered soils of a tropical rain forest: evidence for a potential positive feedback to climate warming

Currently, it is unknown what role tropical forest soils will play in the future global carbon cycle under higher temperatures. Many tropical forests grow on deeply weathered soils and although it is generally accepted that soil carbon decomposition increases with higher temperatures, it is not known whether subsurface carbon pools are particularly responsive to increasing soil temperatures. Carbon dioxide (CO₂) diffusing out of soils is an important flux in the global carbon. Although soil CO₂ efflux has been the subject of many studies in recent years, it remains difficult to deduct controls of this flux because of the different sources that produce CO₂ and because potential environmental controls like soil temperature and soil moisture often covary. Here, we report results of a 5‐year study in which we measured soil CO₂ production on two deeply weathered soil types at different depths in an old‐growth tropical wet forest in Costa Rica. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). Annual soil CO₂ efflux varied between 2.8–3.6 μmol CO₂‐C m^?2 s^?1 (old alluvium) and 3.4–3.9 μmol CO₂‐C m^?2 s^?1 (residual). More than 75% of the CO₂ was produced in the upper 0.5 m (including litter layer) and less than 7% originated from the soil below 1 m depth. This low contribution was explained by the lack of water stress in this tropical wet forest which has resulted in very low root biomass below 2 m depth. In the top 0.5 m CO₂ production was positively correlated with both temperature and soil moisture; between 0.6 and 2 m depth CO₂ production correlated negatively with soil moisture in one soil and positively with photosynthetically active radiation in the other soil type. Below 2 m soil CO₂ production strongly increased with increasing temperature. In combination with reduced tree growth that has been shown for this ecosystem, this would be a strong positive feedback to ecosystem warming.