ORCA

Quantifying terrestrial carbon storage and predicting the sensitivity of ecosystems to climate change relies on our ability to obtain observational constraints on photosynthetic and respiratory activity at large scales (ecosystem, regional and global). Currently, large-scale estimates of photosynthesis and respiration are principally derived from models that use parameterisations of carbon-climate feedbacks that are based on highly uncertain climate sensitivities for photosynthesis and respiration. Here we present a multi-tracer approach to constrain our estimates of photosynthesis and respiration at large scales and better represent these processes in vegetation models. In terrestrial ecosystems, dissolved CO2 in leaf and soil water pools rapidly goes into oxygen isotopic equilibrium with water [CO2+H218OH2O+CO18O]. In leaves, where the enzyme carbonic anhydrase (CA) is present and abundant, this isotopic equilibrium is reached almost instantaneously. As a consequence, and because soil and leaf water pools have different oxygen isotope composition (d18O), CO2 fluxes from leaves and soils carry very distinct d18O signals and can thus be tracked from the fluctuations in the d18O of atmospheric CO2. Recent studies also suggest that carbonyl sulphide (COS) is a good tracer of CO2 transfer into foliage because COS molecules diffuse like CO2 molecules through stomata and also react with CA inside mesophyll cells. However while CO2 reversibly interacts with CA and can diffuse back to the atmosphere before reaching a carboxylation site, COS undergoes an irreversible hydrolysis to form hydrogen sulphide [COS+H2O->H2S+CO2]. Thus, given the heterogeneous distribution of CA in mesophyll cells, the exact relationship between leaf COS, CO18O and CO2 fluxes is still unclear. In addition, as CA is widespread in diverse species from the Archaea, Bacteria and Algae domains, rapid CO2-H2O isotopic exchange and COS hydrolysis may also occur at the soil surface. Soil CA activity was neglected in global CO18O budgets until studies led by the project PI demonstrated the necessity to account for it in order to explain d18O measurements of the net soil CO2 efflux. The project PI also demonstrated that soil CA plays an extremely important role in the global CO18O mass balance and strongly affects the magnitude of global photosynthesis derived with this tracer. Understanding how CA activity is regulated in terrestrial ecosystems is the key for using atmospheric budgets of COS and the d18O of CO2 to constraint the existing parameterisation of CO2 gas exchange embedded in current models and increase the accuracy of our large-scale estimates of photosynthesis and respiration over land. To this end we have designed a project with four objectives. (1) First we will characterise the COS transport and CA activity within the leaf using gas exchange and isotopic tracing techniques on wild species but also mutants with different CA isoforms, in order to better understand how COS and CO2 fluxes are related at the leaf level and respond to environmental factors. (2) We will also characterise simultaneously COS and CO18O uptake by soil monoliths, the relative abundance and community structure of soil bacteria, fungi and algae and their response to controlled moisture and temperature variations in order to quantify precisely soil CA activity from different soil types and identify its main drivers. (3) We will then perform continuous flux measurements of COS and CO18O at the ecosystem and component scales over one growing season in a temperate forest in order to produce a unique, multi-tracer ecosystem-scale dataset to be used as a benchmark for multi-tracer land surface models. (4) Finally we will combine all the theoretical knowledge gained from the other objectives into a coherent modelling framework compatible with global land surface models, in order to provide the theoretical basis for using COS and CO18O as additional tracers of the global CO2 budget.