Global modelling of soil carbonyl sulfide exchanges

Abstract

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.

Ongoing Decline in the Atmospheric COS Seasonal Cycle Amplitude over Western Europe: Implications for Surface Fluxes

“…Mechanistic soil and vegetation COS models have recently been implemented in the ORCHIDEE LSM [27,28]. The implementation of spatio-temporal variations in the nearsurface atmospheric COS concentrations in the modeling of biogenic COS fluxes (Figure 3) helped reduce the imbalance of the atmospheric budget of this gas by lowering its uptake by soils and vegetation globally [28]. A reduction of about 8% in the vegetation uptake in the NH was simulated by the ORCHIDEE LSM between 2016 and 2019 (Figure 3).…”

“…ORCHIDEE global simulations were performed by running a 340-year spin-up simulation in fixed conditions to stabilize all carbon pools and to equilibrate the net biome production [26], followed by a transient phase simulation to introduce increasing CO 2 concentrations, varying climate, and PFT distribution. Next, vegetation and soil COS fluxes were computed from 2000 to 2019 at a 0.5 • spatial resolution following [27,28], respectively. Of central importance in the following is the plant COS uptake, which is based on [29], where the flux in COS uptake (F COS ) is the product of the background atmospheric COS mixing ratio (COS) and gT_COS, where gT_COS is the total of three conductances to COS, i.e., the boundary layer, stomatal, and internal conductances to COS. All fluxes from the different COS source and sink components are listed in Table 1; this global budget of COS is almost in balance (i.e., +19 GgS yr −1 ).…”

Soil Carbonyl Sulfide (OCS) Fluxes in Terrestrial Ecosystems: An Empirical Model

“…(b) Output from the process‐based model in Abadie et al. (2022) integrated into ORCHIDEE. (c) The approach used in Berry et al.…”

“…Using soil temperature and soil moisture from the top 9 cm of ORCHIDEE runs, the average global exchange from our empirical model from 2009 to 2016 is −187 Gg S yr −1 when tundra regions (defined in ORCHIDEE and SOCSEM in Table 4) are assumed to act similarly to a temperate forest floor and −142 Gg S yr −1 when tundra regions act similarly to grasslands, compared to −126 Gg S yr −1 from the process-based model (Abadie et al, 2022).…”

“…In this study, we compare the anoxic OCS production from wetlands calculated using a redox relationship and the land surface model ORCHIDEE in Abadie et al. (2022) to the simple relationship shown in Figure 1b and Table 2. We use the weighted average of the upper 9 cm of the soil column temperature generated by ORCHIDEE to drive our model.…”