Carbonyl sulfide reflections of leaf and ecosystem processes in a tropical rainforest under controlled drought

A promising tracer for partitioning the global balance of CO₂ is carbonyl sulfide (COS or OCS), a trace gas with leaf-level mechanisms shared with carbon dioxide (CO₂) and water (H₂O). COS is therefore used to derive insights into photosynthesis and transpiration at ecosystem to global scales. However, it remains unclear whether COS reflects photosynthesis or stomatal conductance most strongly, as its leaf biochemical and physical processes are not perfectly analogous to either CO₂ or H₂O. There is therefore a need to evaluate the models that encapsulate our current understanding of leaf and soil COS fluxes and predictions of carbon and water cycling against independent constraints in tractable experimental systems.

In this study, we measured ecosystem, leaf, and soil fluxes of COS in the model Biosphere 2 (B2) Tropical Rainforest across a controlled whole ecosystem drought manipulation. We simultaneously, measured the stable isotopes of CO₂, H₂O, and their isotopes (¹³C-CO₂, ¹⁸O-CO₂, ²H-H₂O, ¹⁸O-H₂O) on atmosphere, leaf, and soil measurement streams connected to atmospheric towers, leaf chambers, and soil flux chambers. During the B2 Water, Atmosphere, and Life Dynamics (B2 WALD) campaign, the enclosed ecosystem received no rain for 66 days and was first rewet at depth (2-4 m) at 54 days. Here, we compare COS fluxes to simultaneous and independent measurements of GPP and transpiration from the leaf to ecosystem scales across ecosystem control, drought, and recovery. We further integrate COS measurements with the aforementioned isotopic tracers of carbon and water cycling into the Simple Biosphere Model (SiB3). Our goal is to explore the strengths and limitations of COS as a tracer of ecosystem processes dynamically responding to severe and controlled ecosystem drought.