Soil-atmosphere exchange of carbonyl sulfide in a Mediterranean citrus orchard

Carbonyl sulfide (COS) is used as a tracer of CO ₂ 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 CO ₂ 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 4.6pmolm ^-2 s ^-1 and net emission in sun-exposed soil between rows of up to 2.6pmolm ^-2 s ^-1 , with an overall mean uptake value of 1.1±0.1pmolm ^-2 s ^-1 . Soil COS concentrations decreased with soil depth from atmospheric levels of ∼450 to ∼100ppt at 20cm depth, while CO ₂ concentrations increased from ∼400 to ∼5000ppm. 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 CO ₂ 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.