Quantifying the COS fluxes from plane, willow, beach and oak litter

Flux partitioning, the quantification of photosynthesis and respiration, is a major uncertainty in modelling the carbon cycle and in times when robust models are needed to assess future global changes a persistent problem. A promising new approach is to derive gross primary production (GPP) from measurements of the carbonyl sulfide (COS) flux, the most abundant sulfur-containing trace gas in the atmosphere, with a mean concentration of about 500 pptv in the troposphere. The method is based on the observation that COS and CO2 enter the leaf via a similar pathway and are processed by the same enzyme (carbonic anhydrase), in case of COS a unidirectional process, allowing researchers to use COS uptake as a proxy for the gross CO2 uptake by plants. A prerequisite for using COS as a proxy for photosynthesis is a robust estimation of all non-living-leaf sources and sinks in an ecosystem. One major uncertainty in this regard is the contribution of soils and their respective litter layers to the overall ecosystem COS flux.

COS and CO2 fluxes from litter were measured in real-time using a quantum cascade laser (QCL). The plant litter from four different broadleaf tree species (plane, willow, beech and oak), collected a maximum of one hour before measurements started in the lab (to retain in situ moisture and the microbial biome), was measured under alternating dark and light (UV-A) conditions.

COS litter fluxes varied between the tree species, with plane primarily emitting COS, beech consuming COS and oak and willow being on average neutral (willow with a huge variance). COS litter fluxes within a species seem to correlate with litter moisture. The COS flux was ranging between -4 and 4 pmol kg DW-1 s-1, which is relevant in magnitude compared to the overall ecosystem COS flux and shouldn’t be neglected in future assessments of the global COS budget.