Ecosystem gross CO2 fluxes in a tropical rainforest estimated from carbonyl sulfide (COS)

Carbonyl sulfide (COS) is a promising new tracer to constrain the gross CO₂ fluxes of land ecosystems, particularly in tropical forests where CO₂ flux partitioning is often problematic due to the absence of turbulent flow at night. Since vegetation COS and CO₂ uptake during photosynthesis is closely coupled, the gross fluxes of photosynthesis and respiration can be quantified through the concurrent measurements of COS and CO₂. We measured ecosystem COS and CO₂ exchange over four months in a tropical rainforest at La Selva, Costa Rica. We observed a strong ecosystem uptake of COS with a diel signal that was similar but not identical to net CO₂ fluxes. Soils at the site mostly acted as COS sinks, correlated with soil moisture. The COS and CO₂ data were used to calculate canopy photosynthesis (approx. GPP) from net ecosystem CO₂ exchange (NEE) based on the empirical relationship of leaf relative uptake of COS and CO₂. Mid-day COS-based GPP estimates ranged from -10 to -15 μmol m^-2 s^-1, compared to NEE of -5 to -10 μmol m^-2 s^-1. Ecosystem respiration, calculated as the difference of NEE and GPP, ranged from 5 to 10 μmol m^-2 s^-1, similar to previous estimates of 5 to 9 μmol m^-2 s^-1 from CO₂ flux partitioning and respiration component measurements at the site. Our results support the application of COS as a new tool in ecosystem flux partitioning that may be particularly useful in tropical forests.