Abstract
Abstract. Estimates of photosynthetic and respiratory fluxes at large scales are needed to improve our predictions of the current and future global CO2 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 CO2 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 CO2 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 CO2 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.
Highlights
The terrestrial biosphere is, along with the ocean, the largest sink in the global atmospheric CO2 budget, with a very large year-to-year variability (e.g. Gurney and Eckels, 2011)
There is a scarcity of observations on how photosynthetic gross primary productivity (GPP) and respiration over land respond individually to warmer temperatures, increasing atmospheric CO2 mixing ratios and changes in water availability (Beer et al, 2010; Frankenberg et al, 2011; Welp et al, 2011; Wingate et al, 2009)
Because different soil weights were measured at different times with new soil material each time, it is possible that they would correspond to slightly different fCA values and this could explain the slight mismatch between the model and the fitted curve on the observations
Summary
The terrestrial biosphere is, along with the ocean, the largest sink in the global atmospheric CO2 budget, with a very large year-to-year variability (e.g. Gurney and Eckels, 2011). OCS consumption by soils seems to dominate OCS emission, and its contribution to the atmospheric budget is large, at about one third of the OCS uptake by vegetation, but with a large uncertainty (Berry et al, 2013; Kettle et al, 2002; Launois et al, 2015) This large uncertainty in the OCS exchange rate from soils is partly caused by the variety of approaches used to obtain a global estimate of this flux. We evaluate our model against observed soil OCS uptake rates and patterns from the literature and discuss how the CA-catalysed reaction rates for each soil type can be reconciled with those typically observed for CO2 hydration, given the differences in affinity of CA for OCS and CO2
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
