"The aim of COSIRIS is to isolate the simultaneous fluxes of photosynthesis and respiration of the terrestrial biosphere. With explicit knowledge of the component fluxes, we will: 1) test process based models of photosynthesis and respiration, 2) determine the sensitivity of each flux to environmental conditions, and 3) derive predictions of their responses to climate change. Specifically, COSIRIS aims to build a research facility to integrate a new tracer, carbonyl sulfide (COS) with CO2, water and their stable isotopes in a multi-tracer framework as a tool to separately investigate photosynthesis and respiration. In terrestrial ecosystems, CO2 is often taken up and released at the same time. Similar to CO2, COS is taken up during photosynthesis, but unlike CO2, concurrent COS emissions are small. Parallel COS and CO2 measurements thus promise to provide estimates of gross photosynthetic fluxes – impossible to measure directly at scales larger than a few leaves. The use of COS to derive CO2 fluxes has not been verified yet, but enough is known about their parallel pathways to suggest that COS, CO2 and its isotopes can be combined to yield powerful and unique constraints on gross carbon fluxes. COSIRIS will develop the expertise necessary to achieve this goal by providing: 1. an in-depth analysis of processes involved in COS uptake by vegetation, and of potentially interfering influences such as uptake by soil, 2. a novel process-based multi-tracer modelling framework of COS, CO2, water and their isotopes at the ecosystem scale, 3. extensive datasets on concurrent fluctuations of COS, CO2, water and their isotopes in ecosystems. This innovative approach promises advances in understanding and determining gross carbon fluxes at ecosystem to continental scales, particularly their variations in response to climate anomalies."