Global modelling of soil carbonyl sulfide exchanges
Authors: Abadie Camille , Maignan Fabienne , Remaud Marine , Ogée Jérôme , Campbell J. Elliott , Whelan Mary E. , Kitz Florian , Spielmann Felix M. , Wohlfahrt Georg , Wehr Richard , Sun Wu , Raoult Nina , Seibt Ulli , Hauglustaine Didier , Lennartz Sinikka T. , Belviso Sauveur , Montagne David , Peylin Philippe 
Published: May 2022

Abstract. Carbonyl sulfide (COS) is an atmospheric trace gas of interest for
C cycle research because COS uptake by continental vegetation is strongly
related to terrestrial gross primary productivity (GPP), the largest and
most uncertain flux in atmospheric CO2 budgets. However, to use
atmospheric COS as an additional tracer of GPP, an accurate quantification
of COS exchange by soils is also needed. At present, the atmospheric COS
budget is unbalanced globally, with total COS flux estimates from oxic and
anoxic soils that vary between −409 and −89 GgS yr−1. This uncertainty
hampers the use of atmospheric COS concentrations to constrain GPP estimates
through atmospheric transport inversions. In this study we implemented a
mechanistic soil COS model in the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) land surface model to simulate
COS fluxes in oxic and anoxic soils. Evaluation of the model against flux
measurements at seven sites yields a mean root mean square deviation of 1.6 pmol m−2 s−1, instead of 2 pmol m−2 s−1 when using a previous
empirical approach that links soil COS uptake to soil heterotrophic
respiration. However, soil COS model evaluation is still limited by the
scarcity of observation sites and long-term measurement periods, with all
sites located in a latitudinal band between 39 and
62∘ N and no observations during wintertime in this study. The new
model predicts that, globally and over the 2009–2016 period, oxic soils act
as a net uptake of −126 GgS yr−1 and anoxic soils are a source of
+96 GgS yr−1, leading to a global net soil sink of only −30 GgS yr−1, i.e. much smaller than previous estimates. The small magnitude
of the soil fluxes suggests that the error in the COS budget is dominated by
the much larger fluxes from plants, oceans, and industrial activities. The
predicted spatial distribution of soil COS fluxes, with large emissions from
oxic (up to 68.2 pmol COS m−2 s−1) and anoxic (up to 36.8 pmol COS m−2 s−1) soils in the tropics, especially in India and in the
Sahel region, marginally improves the latitudinal gradient of atmospheric
COS concentrations, after transport by the LMDZ (Laboratoire de Météorologie Dynamique) atmospheric transport model.
The impact of different soil COS flux representations on the latitudinal
gradient of the atmospheric COS concentrations is strongest in the Northern
Hemisphere. We also implemented spatiotemporal variations in near-ground
atmospheric COS concentrations in the modelling of biospheric COS fluxes,
which helped reduce the imbalance of the atmospheric COS budget by lowering
soil COS uptake by 10 % and plant COS uptake by 8 % globally (with a
revised mean vegetation budget of −576 GgS yr−1 over 2009–2016).
Sensitivity analyses highlighted the different parameters to which each soil
COS flux model is the most responsive, selected in a parameter optimization
framework. Having both vegetation and soil COS fluxes modelled within
ORCHIDEE opens the way for using observed ecosystem COS fluxes and larger-scale atmospheric COS mixing ratios to improve the simulated GPP, through
data assimilation techniques.