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Carbonyl sulfide and sun-induced fluorescence as joint constraints on terrestrial carbon cycling

Periodic Reporting for period 1 - COSIF (Carbonyl sulfide and sun-induced fluorescence as joint constraints on terrestrial carbon cycling)

Reporting period: 2018-01-01 to 2019-12-31

Globally, terrestrial ecosystems are presently absorbing around 30 % of the carbon dioxide (CO2) that humans are releasing into the atmosphere every year, thus slowing down global warming. The main driver of the terrestrial carbon sink is gross primary productivity (GPP), the photosynthetic uptake by plants, which is partially counteracted by ecosystem respiration, which returns some of the GPP back to the atmosphere as CO2. GPP, unfortunately, cannot be measured directly and thus needs to be inferred from models, which are constrained by empirical data. The resulting uncertainty impedes our ability to project whether terrestrial ecosystems will be sequestering CO2 at similar rates in the future or whether their sink capacity might saturate or even decline. The MSCA project COSIF aims at reducing the uncertainty of GPP estimates by using two novel constraints of GPP, the ecosystem uptake of the trace gas carbonyl sulfide (COS) and the solar-induced chlorophyll fluorescence (SIF) in a model-data integration approach. These two proxies of GPP target different components of the photosynthesis process and thus allow constraining GPP from different perspectives. The reduced uncertainty of future GPP estimates will have far reaching consequences for the design of decarbonization pathways required for reaching defined climate goals and thus affect the global society.
The project involved three multi-month field campaigns during which concurrent ecosystem-scale CO2 and COS eddy covariance flux measurements were jointly conducted with active and passive chlorophyll fluorescence measurements. The study sites were selected to cover differently structured ecosystems (grassland, savanna and forest) under different background conditions (rapid growth, heat stress, cold stress). The resulting data were analyzed, have been partially published and further publications are presently in preparation. A process-based modelling framework has been adapted to include COS exchange and is presently used to quantify the additional information content of different multiple constraints on the magnitude and uncertainty of inferred GPP.
Major new insights from COSIF include (i) the observation that dead plant organic matter covering soils may turn ecosystems to source of COS during periods of low soil water content combined with high temperatures and in such conditions soil contribution has to be taken into account when using the ecosystem-scale COS fluxes as a proxy of GPP and (ii) that SIF may show an ambiguous relationship to GPP under conditions of stress and that in such situations non-photochemical quenching of radiation energy absorbed by plant chlorophyll needs to be quantified jointly with SIF in order to reliably estimate GPP.
The inter-annual variability of terrestrial carbon fluxes is characterized by complex climate feedbacks of terrestrial biospheric exchanges. In particular, extreme climate events have been shown to have the potential to contribute to a positive climate-carbon cycle feedback, to decrease carbon uptake by terrestrial ecosystems or even to release carbon to the atmosphere. In this context, understanding and quantification of different ecosystem responses to a changing climate, and in particular to extreme events, is a key target for the scientific community, in order to deliver more accurate climate services to support the accountability of actions undertaken to meet the long-term goals of international treaties and initiatives. Such initiatives are aiming at enhancing the climate change mitigation/adaptation potential of land ecosystems. To this end, one of the main impacts of the COSIF project was the enhanced understanding of the response of different ecosystems to climate extremes.

Another important impact was the advanced integration of different novel measurement techniques. This project was, to the best of our knowledge, the first to make use of joint active and passive chlorophyll fluorescence, and CO2/COS measurements to diagnose the functioning of various ecosystems and their response to climate variables.
Moreover, the COSIF project provided new insights on using SIF and COS as GPP proxies, which are crucial for further developing of both methods and will thus provide more reliable future GPP estimates required for designing and implementing decarbonization strategies.
The WhiteRef and SIF system jointly measuring hyperspectral reflectance and SIF at Neustift.
In-situ cross-calibration of SIF systems, Majadas, Spain.
Monitoring of Scots pine forest in Mezyk, Poland. In the foreground: active PAM fluorescence.
Above canopy EC COS measurements in Majadas, Spain.