Articles | Volume 8, issue 10
Geosci. Model Dev., 8, 3055–3070, 2015
Geosci. Model Dev., 8, 3055–3070, 2015

Model description paper 02 Oct 2015

Model description paper | 02 Oct 2015

A soil diffusion–reaction model for surface COS flux: COSSM v1

W. Sun1, K. Maseyk2,a, C. Lett2,b, and U. Seibt1,2 W. Sun et al.
  • 1Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA
  • 2Institute of Ecology and Environmental Sciences, Université Pierre et Marie Curie, Paris, France
  • anow at: The Open University, Milton Keynes, UK
  • bnow at: British Antarctic Survey, Cambridge, UK

Abstract. Soil exchange of carbonyl sulfide (COS) is the second largest COS flux in terrestrial ecosystems. A novel application of COS is the separation of gross primary productivity (GPP) from concomitant respiration. This method requires that soil COS exchange is relatively small and can be well quantified. Existing models for soil COS flux have incorporated empirical temperature and moisture functions derived from laboratory experiments but not explicitly resolved diffusion in the soil column. We developed a mechanistic diffusion–reaction model for soil COS exchange that accounts for COS uptake and production, relates source–sink terms to environmental variables, and has an option to enable surface litter layers. We evaluated the model with field data from a wheat field (Southern Great Plains (SGP), OK, USA) and an oak woodland (Stunt Ranch Reserve, CA, USA). The model was able to reproduce all observed features of soil COS exchange such as diurnal variations and sink–source transitions. We found that soil COS uptake is strongly diffusion controlled and limited by low COS concentrations in the soil if there is COS uptake in the litter layer. The model provides novel insights into the balance between soil COS uptake and production: a higher COS production capacity was required despite lower COS emissions during the growing season compared to the post-senescence period at SGP, and unchanged COS uptake capacity despite the dominant role of COS emissions after senescence. Once there is a database of soil COS parameters for key biomes, we expect the model will also be useful to simulate soil COS exchange at regional to global scales.

Short summary
We report a soil COS flux model that is the first to resolve both vertical transport and microbial sources and sinks in soil and litter. By evaluation with field data, we show that the model can reproduce observed daily and long-term variations of soil COS flux. We also demonstrate that diffusion is important in controlling the flux, by limiting the COS available for soil uptake when there is strong litter uptake and modulating the water content dependence of soil uptake.