The vegetated terrestrial surface plays an outsized role in global carbon cycling, and annual variability in CO2 removal from the atmosphere is highly dependent on the strength of the terrestrial carbon sink. However, despite large advances in measurement and modelling techniques, and increased spatiotemporal measurements of [CO2], global estimates of terrestrial photosynthesis (Gross Primary Productivity or GPP) vary between 90- 175 Pg C yr-1. This is due to our inability to measure photosynthesis beyond the leaf level. The eddy covariance method is widely used to measure net uptake of CO2 (NEE) across a range of ecosystems but lacks a process-based approach to partition this into source (Respiration) and sink (GPP) terms.
Atmospheric measurements of carbonyl sulphide (OCS) have recently been shown to provide an independent and direct estimate of plant productivity. OCS is the most abundant reduced sulphur gas present in the atmosphere, with a mean atmospheric concentration of ~ 500 ppt (parts per trillion) and is emitted into the atmosphere from oceans via direct emissions or oxidation of CS2, and consumed by leaves of actively photosynthesising leaves.
In this study, co-authors and I present the first measurements of OCS from a well studied temperate oldgrowth coniferous forest, focussing on biotic and abiotic controls on ecosystem OCS flux. Estimates of GPP from OCS flux are then compared with other independent estimates from the site.
We find that eddy-flux based measurements of CO2 flux underestimate GPP at the site, likely driven by to an underestimation of respiration. We suggest that measurements of trace gases such as OCS, coordinated with physiological and structural measurements, may jointly provide constraints on GPP.