Using Atmospheric COS–CO₂ Seasonal Amplitude Ratios to Quantify C4 Contributions to GPP

Enhanced photosynthetic CO₂ uptake (gross primary productivity; GPP) by terrestrial plants in response to rising atmospheric CO₂ concentrations constitutes the largest and most uncertain ecosystem feedback on climate. Direct measurement of GPP at scales above the leaf is not possible due to co-occurring respiratory fluxes. Carbonyl sulfide (COS) has emerged as a promising tracer of GPP across scales because of its predominantly one-way flux into leaves and the use of leaf relative uptake (LRU) to convert COS uptake into GPP. However, differences in the relative uptake of COS to CO₂ between C3 and C4 vegetation across scales must be accounted for in the application of COS as a tracer and could potentially be applied to detect climate-driven shifts in C3/C4 vegetation distributions. These aspects have largely been ignored, with most COS measurements focusing on C3 vegetation, and only limited C4 COS measurements available.

Here, we develop an atmospheric-based approach to identify and quantify C4 vegetation contributions to large-scale photosynthetic uptake using existing COS and CO₂ concentration measurements from multiple NOAA Global Monitoring Laboratory (GML) network sites. We analyze the seasonal cycle amplitudes of COS as a function of CO₂, defined as atmospheric relative uptake (ARU), and identify sites that deviate from the regression line describing the majority of sites. We derive an analytical framework linking leaf-, ecosystem-, and atmospheric-scale relative uptake, explaining how shifts in physiological traits characteristic of C3 and C4 vegetation influence atmospheric COS and CO₂ signals. Using this framework together with gridded fluxes from the land surface model: Simple Biosphere Model (version 4; SiB4), we show that sites influenced by C4 vegetation exhibit systematic deviations in ARU relative to predominantly C3-influenced sites. These results demonstrate that ARU provides a viable means of detecting and quantifying C4 vegetation contributions to GPP at regional scales (10² – 10³ km²), and can be used to detect climate-driven shifts in C3/C4 distributions. Our study advances the use of COS as a tracer of GPP and of C3 and C4 photosynthesis across scales.