Soil carbonyl sulfide exchange

Gross primary productivity (GPP), that is the gross uptake of carbon dioxide (CO2) by plants, is the proximate driver of the contemporary land carbon sink. Unfortunately, GPP, at least on the ecosystem-scale, cannot be measured directly and instead indirect approaches need to be used to approximate GPP. Among these, ecosystem-scale flux measurements of carbonyl sulfide (COS or OCS) have been proposed recently as a novel, and possibly more robust, approach for estimating GPP. Among the many assumptions underlying this approach, a major one is related to the presence of non-leaf sinks or sources of COS within an ecosystem ideally these would be negligible or directly measurable or predictable on the basis of readily available ancillary measurements. The most important non-leaf sources/sinks for COS are soils. During the past couple of years it has become clear that COS, driven by biotic and abiotic processes, may both be consumed and produced in soils, the relative magnitude of which (together with the ambient concentration and soil diffusivity) determines the net exchange across the soil- atmosphere interface. To date, however, no consensus has been reached in terms of which circumstances cause a soil to represent a net sink or a net source and also the magnitude of available soil COS source/sink estimates varies greatly between studies. In support of the use of COS flux measurements as proxies for GPP, it is the overarching aim of the proposed project to arrive at generalizable conclusions regarding where, when and why soil COS consumption prevails over production (or vice versa) and how this affects the exchange across the soil-atmosphere interface. To this end we propose an experimental approach which quantifies biotic and abiotic contributions to the soil COS flux across the major soil types of the Earth, investigates the hitherto neglected contribution of roots to the soil COS exchange and merges all these lines of evidence into an extant model which will be tested against in situ field measurements.

Gross primary productivity (GPP), that is the gross uptake of carbon dioxide (CO2) by plants, is the proximate driver of the contemporary land carbon sink. Unfortunately, GPP, at least on the ecosystem-scale, cannot be measured directly and instead indirect approaches need to be used to approximate GPP. Among these, ecosystem-scale flux measurements of carbonyl sulfide (COS or OCS) have been proposed recently as a novel, and possibly more robust, approach for estimating GPP. Among the many assumptions underlying this approach, a major one is related to the presence of non-leaf sinks or sources of COS within an ecosystem ideally these would be negligible or directly measurable or predictable on the basis of readily available ancillary measurements. The most important non-leaf sources/sinks for COS are soils. During the past couple of years, it has become clear that COS, driven by biotic and abiotic processes, may both be consumed and produced in soils, the relative magnitude of which determines the net exchange across the soil-atmosphere interface. To date, however, no consensus has been reached in terms of which circumstances cause a soil to represent a net sink or a net source and also the magnitude of available soil COS source/sink estimates varies greatly between studies. In support of the use of COS flux measurements as proxies for GPP, it was the overarching aim of the proposed project to arrive at generalizable conclusions regarding where, when and why soil COS consumption prevails over production (or vice versa) and how this affects the exchange across the soil-atmosphere interface. To this end we conducted a series of experiments on soils with and without plant roots, with common fungi isolated from soil samples, and with litter of various plant species. Our results show that the presence of plant roots leads to an uptake of COS, which has important implications for global soil COS exchange budgets, which are largely based on measurements from root-free sieved soil samples. Furthermore, our results suggest that the COS exchange of common soil fungi is species-specific and thus the composition of the soil fungal community is likely to have implications for the soil COS exchange. Finally, our results demonstrate that UV radiation causes a release of COS from plant litter, which in turn has implications for how models simulate soil COS exchange, requiring these to simulate the quantity and quality of the canopy- penetrating solar radiation in order to reliably estimate the COS emission from plant litter at the soil surface.