Land Surface-Atmosphere CO2 Fluxes in the Critical Zone

The terrestrial carbon cycle is rooted in the Critical Zone, as its main processes involve matter and energy exchanges between vegetation, soil, water, rock and the atmosphere. The biological carbon cycle is, together with the water cycle, a crucial element cycle for the regulation of life and climate on Earth. Soils and the terrestrial carbon cycle are intimately linked, as none of them would exist without the other: primitive soils were formed by the action of the first terrestrial photosynthet- ically active organisms, and today soils support vegetation on Earth and represent the greatest carbon pool of the terrestrial biosphere, containing much more carbon than the atmosphere. Carbon is exchanged mainly through CO2 fluxes, which take place at the soil–vegetation–atmosphere interface and whose magnitude is influ- enced by the characteristics of the soil, of climate and of ecosystems. Photosynthesis and respiration from terrestrial vegetation and soils represent the two major compo- nents of carbon fluxes between the atmosphere and the biosphere. Their balance (or imbalance) regulates the evolution of climate on Earth and is in turn influenced by the climatic conditions. The disturbances caused by human activities (e.g. fossil fuels emissions, land use changes, fires) are now altering this balance. Measuring and modelling this cycle in different ecosystem types and under different levels of human pressures is necessary to deepen our understanding of the dynamics of the Earth life support system and preserve the services it provides to the benefit of humankind.