Emission of CO2 from tropical riparian forest soil is controlled by soil temperature, soil water content and depth to water table

Tropical forests play a key role in the global carbon cycle. However, little is known about carbon cycling in the substantial portion of tropical forests that are low-lying, with shallow and fluctuating water tables. This study aimed to determine what factors control emissions of CO2 from soil in a riparian rainforest in Queensland, Australia. Emissions were measured over the course of 1 year, using static chambers. Emission rates were significantly related to soil temperature (0–0.1 m depth), soil water content (0–0.12 m depth) and depth to water table. The most efficient linear model of emissions as a function of measured parameters, which also included soil pH (0–0.1 m depth), had r2 = 0.355. CO2 emissions were highest (5.2–7.5 µmol m–2 s–1) at moderate soil temperature (24-28°C), water table depth (0.2–1.5 m) and soil water-filled porosity (0.25–0.79). They were lowest (<0.5 µmol m–2 s–1) at low soil temperature (<22°C) or when the water table was within 0.15 m of the surface. An additional interaction between temperature and soil water was determined in the laboratory. Incubation of soil cores showed that temperature sensitivity of the heterotrophic component of respiration increased as the soil dried. It is clear that models of soil respiration in lowland tropical forests should take into account depth to water table, which is a key, but hitherto unreported, controller of CO2 emissions in tropical forests.