Consumption and production of carbon monoxide in soils: A global model analysis of spatial and seasonal variation

Abstract

We have developed a simulation model to estimate net seasonal emission of carbon monoxide (CO) from soils world-wide which implies a smaller biogenic sink for CO than previously surmised. Field measurements have suggested that soils can play an important regional role in the net terrestrial exchange of carbon monoxide with the atmosphere. Previous global estimates of the net soil sink for atmospheric CO have been made by multiplying averages of small chamber measurements for various soil or vegetation classes by estimates of the area covered by each class. Simulation models driven by gridded databases can also contribute to global flux estimates. Such models are useful for evaluation of potential effects of changes in climate and land use, and for identification of weaknesses in both data and mechanistic understanding. We applied a modified version of Fick's first law based on computations for diffusivity in aggregated media, together with a soil water balance model run on a 1° global grid, to make independent estimates of CO uptake by soils worldwide. Unlike previous global assessments, we assume that gross uptake rates are negligible in very dry desert soils (that are mostly devoid of microbial activity), in frozen soils, and in wetlands. The model results support a reference case estimate of 16 Tg CO yr −1 for gross consumption of atmospheric CO in soils worldwide. However, owing to uncertainties in the seasonal boundary conditions and the actual soil depth for CO consumption activity, we estimate that this reference case for gross CO consumption in soils could go as high as 50 Tg CO yr −1 globally. We also estimated production of CO from decaying soil organic matter as a process of chemical oxidation. Our model for gross production of CO from surface soils supports a global reference case flux of 9.4 ± 2.5 Tg CO yr −1. Combination of gross consumption and production fluxes implies a global net CO uptake flux of about 7–40 Tg CO yr −1 in soils. Model results also support the hypothesis that temperate dry zones are the primary global sinks for soil CO, whereas tropical wet zones are the primary global sources for soil CO production. Notably, there is an order-of-magnitude difference between our model estimates and previous sink calculations based on extrapolations of measured fluxes, which suggests the need for more extensive soil CO flux studies, especially in remote regions where biogenic emissions could potentially exceed industrial sources of CO.