A new mechanistic framework to predict OCS fluxes from soils

Abstract

Abstract.Estimates of photosynthetic and respiratory fluxes at large scales are needed to improve our predictions of the current and future global CO 2 cycle. Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere and has been proposed as a new tracer of photosynthetic gross primary productivity (GPP), as the uptake of OCS from the atmosphere is dominated by the activity of carbonic anhydrase (CA), an enzyme abundant in leaves that also catalyses CO 2 hydration during photosynthesis. However soils also exchange OCS with the atmosphere, which complicates the retrieval of GPP from atmospheric budgets. Indeed soils can take up large amounts of OCS from the atmosphere as soil microorganisms also contain CA, and OCS emissions from soils have been reported in agricultural fields or anoxic soils. To date no mechanistic framework exists to describe this exchange of OCS between soils and the atmosphere, but empirical results, once upscaled to the global scale, indicate that OCS consumption by soils dominates OCS emission and its contribution to the atmospheric budget is large, at about one third of the OCS uptake by vegetation, also with a large uncertainty. Here, we propose a new mechanistic model of the exchange of OCS between soils and the atmosphere that builds on our knowledge of soil CA activity from CO 2 oxygen isotopes. In this model the OCS soil budget is described by a first-order reaction-diffusion-production equation, assuming that the hydrolysis of OCS by CA is total and irreversible. Using this model we are able to explain the observed presence of an optimum temperature for soil OCS uptake and show how this optimum can shift to cooler temperatures in the presence of soil OCS emission. Our model can also explain the observed optimum with soil moisture content previously described in the literature as a result of diffusional constraints on OCS hydrolysis. These diffusional constraints are also responsible for the response of OCS uptake to soil weight and depth observed previously. In order to simulate the exact OCS uptake rates and patterns observed on several soils collected from a range of biomes, different CA activities had to be invoked in each soil type, coherent with expected physiological levels of CA in soil microbes and with CA activities derived from CO 2 isotope exchange measurements, given the differences in affinity of CA for both trace gases. Our model can be used to help upscale laboratory measurements to the plot or the region. Several suggestions are given for future experiments in order to test the model further and allow a better constraint on the large-scale OCS fluxes from both oxic and anoxic soils.

Soil fluxes of carbonyl sulfide (COS), carbon monoxide, and carbon dioxide in a boreal forest in southern Finland

“…The moisture dependence of COS uptake activity typically manifests as a bell-shaped curve with a moisture optimum (Kesselmeier et al, 1999;Van Diest and Kesselmeier, 2008;Whelan et al, 2016). Below the moisture optimum, microbial uptake of COS is limited by water availability, whereas above it, COS uptake is limited by the diffusional supply of COS from the atmosphere because soil gas diffusivity decreases with moisture content (Sun et al, 2015;Ogée et al, 2016). In this study, the decrease of COS uptake with increasing soil moisture (Fig.…”

“…4 and 5), simulating soil COS uptake in this boreal forest will rely on using soil respiration as an important statistical predictor. In this case, the parameterization scheme used in Berry et al (2013) and the empirical relationship based on the soil relative uptake ratio (COS uptake to CO 2 emission ratio normalized by their concentrations) as in Berkelhammer et al (2014) will be useful in predicting COS uptake, provided that diffusion in the soil column is properly resolved (e.g., Sun et al, 2015;Ogée et al, 2016).…”

“…Similar to other ecosystems, we hypothesize that soil temperature, moisture, and microbial activity are the main drivers of soil COS flux in boreal forests, and they have responses unique to this type of ecosystem. Despite limited knowledge of soil CO processes, we expect similarities in the responses of soil CO flux to soil physical variables and microbial activity compared to the responses of COS flux, based on the reactive transport mechanism in the soil column (Sun et al, 2015;Ogée et al, 2016;Yonemura et al, 2000b). Here we report continuous field measurements of soil COS, CO, and CO 2 fluxes in a Scots pine forest in southern Finland over the late growing season (July to November).…”

The role of soil pH on soil carbonic anhydrase activity

“…24 and 80 mg L −1 were used. We chose these concentration because they would correspond to the upper range of CA concentrations expected in natural soils, assuming a cytoplasmic CA concentration of 0.1 mM (Ogée et al, 2016). Apart from this addition of CA into the irrigation water, all other preparation steps of the soil microcosms were kept identical to the ones described above for the microcosms without CA addition.…”

“…Following the same reasoning as in Ogée et al (2016) for carbonyl sulfide (OCS) hydrolysis, the soil CO 2 hydration rate can also be expressed as a function of bulk CA concentration [CA] (mol m −3 ):…”