Carbonyl sulphide uptake by a terra firme forest in the central Amazon

Carbonyl sulphide (OCS) can be used as a tracer of stomatal opening and gross primary productivity. Its application at the ecosystem level depends on understanding atmospheric transport and whether other sources and sinks are present in the environment of interest. Very little is known about the OCS cycle in widespread terra firme forests of the central Amazon. To address this knowledge gap, we are making concentration measurements of OCS, atmospheric conditions and net CO₂ exchange at various heights on an 80 m tower at the Amazon Tall Tower Observatory in Amazonas, Brazil. These observations are complimented by measurements of OCS exchange from soil flux chambers at the base of the tower. Gradients in atmospheric concentration indicate that the forest within the tower footprint is generally a net sink for OCS. However, forest fires can be a significant source of OCS to the atmosphere during the dry season. Net uptake of OCS varies diurnally supporting the response of stomatal opening to photo-period as the principal control on exchange. However, interpretation of this uptake is complicated by atmospheric transport dynamics and soil activity. The combination of a tall canopy and diurnal variations in atmospheric stability means changes in storage exert a large influence on net exchange at sub-daily timescales. At longer timescales these storage fluxes appear to cancel out, simplifying the estimation of average uptake. Similarly, soil OCS uptake represents a significant and variable proportion of net exchange that needs to be considered when estimating the contribution of photosynthesis.