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

Carbonyl sulphide has potential as a tracer of gross primary productivity. However, its use at the ecosystem scale requires us to understand something about its atmospheric transport and the distribution of sources and sinks within the environment of interest. Despite the importance of understanding the controls on carbon uptake and release by Amazonian forests, very little is known about the carbonyl sulphide cycle of widespread terra firme ecosystems.

 

Here we report on carbonyl sulphide exchange estimated from concentration measurements of carbonyl sulphide at various heights, atmospheric conditions and net carbon dioxide exchange on an 80 m tower, and from soil flux chambers at the Amazon Tall Tower Observatory. The landscape surrounding the measurement site, 150 km north-east of the Brazilian city of Manaus, is typical of the central Amazon consisting of plateaus and steep valleys. Growing on highly weathered and well-drained Ferralsols, these plateaus are covered by old-growth, terra firme forests reaching 30 to 35 m in height. The region experiences relatively stable temperatures throughout the year, but pronounced seasonality in rainfall with a minimum in August and maximum in March.

 

Atmospheric measurements suggest that the forest within the tower footprint is generally a net sink for carbonyl sulphide. However, fires likely represent a regionally significant source of carbonyl sulphide during the dry season. Net uptake of carbonyl sulphide is greater during the day than the night indicating a strong link to light control of stomatal opening. Estimating gross primary productivity from this uptake is complicated by transport dynamics and soil activity. The tall canopy and diurnal variations in atmospheric mixing, with overnight drawn down followed by entrainment of the upper atmosphere after dawn, means storage has a large influence on net exchange at sub-daily timescales. At longer timescales these exchanges appear to cancel out, simplifying the estimation of average uptake. Similarly, uptake of carbonyl sulphide by the soil represents a significant and variable proportion of the estimated net exchange that needs to be considered when estimating the contribution of photosynthesis.