Soil properties in tropical montane cloud forests influence estimates of soil CO₂ efflux

Soils in tropical montane cloud forests contain large amounts of organic carbon but few data are available on the flux of carbon dioxide (CO₂) from them. Routine methods used to measure soil CO₂ efflux, however, may be susceptible to measurement artifact in these ecosystems due to the high porosity and low bulk density of their soils. Here, we measured soil CO₂ efflux in two tropical montane cloud forests, one lowland tropical forest and a garden lawn in the Peruvian Andes-Amazon and used 'swap tests' (repeated alternating measurements) to explore sources of measurement artifact when using two different closed-dynamic chamber systems (made by LI-COR Biosciences, NE, USA): a 'multiplexed system', consisting of 104-long term chambers, 8150-multiplexer and 8100-infra-red-gas analyzer (IRGA); and a 'survey system', consisting of a 20cm-survey chamber and 8100-IRGA. We found that, in the cloud forests, soil CO₂ efflux was significantly higher (by 54±2%) when measured using the multiplexed system compared to when using the survey system. In contrast, there was no significant difference in soil CO₂ efflux measured by the two systems in the lowland tropical forest, on the garden lawn, and in the cloud forests when the soil collars were inserted in the mineral soil. A series of diagnostic tests indicated that the multiplexed system overestimated soil CO₂ efflux in the cloud forests by promoting mass-flow of CO₂ from the organic soil of low bulk density. The measurement artifact was significantly reduced by reducing flow rates and diverting chamber air in-flow away from the soil surface, suggesting that the artifact was due to turbulence within the chamber ('Venturi effects'). Future studies of soil CO₂ efflux in tropical montane cloud forests should be undertaken with consideration of these artifacts, and gas flux monitoring systems should be tested in conditions to represent such environments.