Abstract
Abstract. Carbonyl sulfide (COS) is used as a tracer of CO2 exchange at the ecosystem and larger scales. The robustness of this approach depends on knowledge of the soil contribution to the ecosystem fluxes, which is uncertain at present. We assessed the spatial and temporal variations in soil COS and CO2 fluxes in a Mediterranean citrus orchard combining surface flux chambers and soil concentration gradients. The spatial heterogeneity in soil COS exchange indicated net uptake below and between trees of up to 4.6 pmol m−2 s−1 and net emission in sun-exposed soil between rows of up to 2.6 pmol m−2 s−1, with an overall mean uptake value of 1.1±0.1 pmol m−2 s−1. Soil COS concentrations decreased with soil depth from atmospheric levels of ∼450 to ∼100 ppt at 20 cm depth, while CO2 concentrations increased from ∼400 to ∼5000 ppm. COS flux estimates from the soil concentration gradients were, on average, -1.0±0.3 pmol m−2 s−1, consistent with the chamber measurements. A soil COS flux algorithm driven by soil moisture and temperature (5 cm depth) and distance from the nearest tree, could explain 75 % of variance in soil COS flux. Soil relative uptake, the normalized ratio of COS to CO2 fluxes was, on average, -0.4±0.3 and showed a general exponential response to soil temperature. The results indicated that soil COS fluxes at our study site were dominated by uptake, with relatively small net fluxes compared to both soil respiration and reported canopy COS fluxes. Such a result should facilitate the application of COS as a powerful tracer of ecosystem CO2 exchange.
Highlights
Carbonyl sulfide (COS) is a sulfur-containing analogue of CO2 that is taken up by vegetation following a similar pathway to CO2, hydrolyzed in an irreversible reaction with carbonic anhydrase
The COS fluxes showed systematic uptake under tree (UT), moderate uptake and some emissions between trees (BT), and relatively more emission in the exposed area between rows (BR), with diurnal mean values across seasons of −3.0 ± 0.1, −0.4 ± 0.14 and +0.1 ± 0.1 pmol m−2 s−1, respectively
Soil COS fluxes showed both changes in rates and shifts from net uptake to net emission, with the site hierarchy differing in the different seasons (Fig. 1)
Summary
Carbonyl sulfide (COS) is a sulfur-containing analogue of CO2 that is taken up by vegetation following a similar pathway to CO2, hydrolyzed in an irreversible reaction with carbonic anhydrase It holds great promise for studies of photosynthetic CO2 uptake (Asaf et al, 2013; Berry et al, 2013; Wehr et al, 2017; Whelan et al, 2018). Even for the same soil, COS fluxes could show large variations and both uptake and emission with sensitivities to soil moisture and ambient COS concentrations (Bunk et al, 2017; Kaisermann et al, 2018) These studies assessed the response of COS exchange to environmental controls, e.g., soil moisture and temperature and solar radiation
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