Soil Carbonyl Sulfide (OCS) Fluxes in Terrestrial Ecosystems: An Empirical Model

Abstract

Most land uptake of carbonyl sulfide (OCS) is physically linked to gross primary production (GPP), the largest flux in the global carbon cycle (Berry et al., 2013;Protoschill-Krebs et al., 1995). However, outstanding questions remain that prevent the straightforward use of OCS as a tracer for GPP. In particular, there is a significant budget gap because the anticipated OCS sinks from plants, soils, and oxidation in the atmosphere are larger than all of the sources we have currently accounted for. Estimates of the missing source vary from 230 to 800 Gg S y −1 (Lennartz et al., 2017). One possibility is that we have overestimated the plant sink although it would require an order of magnitude overestimation to account for the entire budget gap, unlikely based on laboratory and field data (Whelan et al., 2018). Two other possibilities have generated more debate: a large source in the tropical Abstract Measurements of carbonyl sulfide (OCS) enable independent estimates of regional stomatal conductance provided that non-stomatal OCS fluxes are well constrained. OCS is taken up through plant leaves, following the same pathway as CO 2 ; in contrast to CO 2 , OCS is irreversibly destroyed in plant leaves and plants do not typically exhibit OCS emissions. Ecosystem uptake of OCS can indicate changes in stomatal opening.Here we present an empirical model to assess the potential impact of soil OCS exchange, the non-Stomatal OCS exchange Empirical Model (SOCSEM, version 0). We created biome-specific response curves characterizing soil OCS exchange and restricted the model design to require only knowledge of soil moisture and surface temperature because remote sensing observations are available for these two features. Comparing the model to field-based chamber observations reveal deviations that can be attributed to missing complexity of the ground surface (having excluded litter and plants without regulated stomata), shortwave radiation, or the soil environment. For agricultural regions with known net emissions, we use remotely-sensed surface temperature data and demonstrate that data resolution can affect anticipated fluxes. We further investigate the influence of regions with unknown soil OCS responses, for example, Arctic tundra. We compare our model to a process-based and respiration-based soil OCS exchange model that has been implemented in a land surface model. Further field study of tropical and arctic ecosystems in conjunction with studies of non-stomatal surfaces in addition to soil (e.g., bryophytes) will increase confidence in applying OCS as a regional tracer for stomatal conductance.Plain Language Summary Carbonyl sulfide (OCS) uptake over ecosystems is a good proxy for land carbon uptake via photosynthesis when non-plant OCS exchange is small or at least knowable. From what we have observed, soil OCS fluxes are typically overwhelmed by plant-based OCS uptake except in the case of wetland soils or when agricultural fields have become hot and dry. In this paper we constructed a model that estimates where soil OCS ...

Root and rhizosphere contribution to the net soil COS exchange

“…Throughout most of the experiment (except in February), both treatments were a source of COS, which is in contrast to many previous lab and field studies (see Whelan et al ( 2018 ) and Whelan et al ( 2022 ) characterizing oxic soils as a COS sink. This is most likely due to the very nutrient rich potting soil used in the experiment, which is commonly used as a cultivation soil in gardening and cannot be compared to natural soils.…”

Optimizing the terrestrial ecosystem gross primary productivity using carbonyl sulfide (COS) within a “two-leaf” modeling framework

“…We took the soil COS modeling scheme including the parameterizations from Abadie et al (2022) and Whelan et al (2022) in this study (see Appendix A2) given that our focus is the COS and GPP relationships and the previous studies have verified this approach over multiple sites with measurements.…”

“…The maximum biotic COS uptake 𝐹𝐹 𝑜𝑜𝑝𝑝𝑠𝑠 and the biotic COS uptake 𝐹𝐹 𝑆𝑆𝑆𝑆𝐶𝐶 𝑔𝑔 are the COS fluxes (pmol m −2 s −1 ) at optimum soil water content 𝑅𝑅𝑆𝑆𝐶𝐶 𝑜𝑜𝑝𝑝𝑠𝑠 and a secondary soil water content 𝑅𝑅𝑆𝑆𝐶𝐶 𝑔𝑔 , and 𝑅𝑅𝑆𝑆𝐶𝐶 𝑔𝑔 > 𝑅𝑅𝑆𝑆𝐶𝐶 𝑜𝑜𝑝𝑝𝑠𝑠 . A more detailed description of the modeling of 𝐹𝐹 𝑐𝑐𝑜𝑜𝑠𝑠,𝑏𝑏𝑠𝑠𝑜𝑜𝑠𝑠𝑠𝑠𝑐𝑐 and the parameterization scheme adopted in this study can be found in Whelan et al (2022).…”

“…In contrast, the process-based model that mechanistically simulates COS plant uptake by incorporating stomatal transport processes were also developed and widely evaluated (Maignan et al, 2021;Kooijmans et al, 2021). Concurrently, the significance of soil COS exchange has also been recognized, leading to the development of a suite of empirical or mechanistic COS soil exchange models (Kesselmeier et al, 1999;Berry et al, 2013;Launois et al, 2015;Sun et al, 2015;Ogée et al, 2016;Whelan et al, 2022). Process-based COS plant uptake model and soil exchange model have been integrated into land surface models (LSMs) (Berry et al, 2013;Maignan et al, 2021;Kooijmans et al, 2021;Chen et al, 2023).…”

Isotope measurements of carbonyl sulfide

“…Biomass burning, both anthropogenic and natural, is also a small source of COS (Stinecipher et al, 2019;Zumkehr et al, 2018), with an estimated emission of 136 GgS yr −1 . COS sinks include the before-mentioned large plant uptake, and a smaller and less well characterized soil uptake flux (Whelan et al, 2018), although some (agricultural) soils have been reported to actually emit COS (Whelan et al, 2022). Together, plant and soil COS uptake have an estimated flux of -1053 GgS yr −1 .…”