Linking the atmosphere and terrestrial biosphere carbon and water cycles using oxygen isotopes.

The rise in the concentration of carbon dioxide (CO2) in the atmosphere over the last two centuries results mainly from the burning of fossil fuel. Recent emissions of CO2 are as much as 9 billion tonnes of carbon annually - almost equivalent to one tonne for every human alive on the planet. This rise is alarming, because increased CO2 in the atmosphere contributes to global warming and climate change. Terrestrial ecosystems, especially forests, can help to reduce the rise of CO2 in the atmosphere by storing carbon in wood and soil. But terrestrial ecosystems also emit CO2 during respiration, releasing almost 20 times more CO2 annually than that released by burning fossil fuels! However, photosynthetic uptake of CO2 compensates for respiration losses, so that the storage of carbon in biomass and soil sinks usually increases in forests over time. Understanding the impact of terrestrial ecosystems on CO2 concentrations in a changing climate requires a better knowledge of the balance between photosynthesis and respiration - for instance, in the summer of 2003, it has been suggested that forests changed from sinks to sources of CO2. In this proposal we are trying to understand the 'ins and outs' of CO2 in the air above the trees, and in the air above the soil. For that, we use the fact that when CO2 exchanges with leaves and soils it swaps one oxygen atom (O) with oxygen in the surrounding water. There are two types of oxygen atoms, a heavy and a light isotope (18O and 16O), and that soil water and leaf water differ in the proportion of these isotopes, due to preferential evaporation of the lighter isotope during evapotranspiration from leaves. Thus, the oxygen exchange between CO2 and water provides a unique insight into the balance between photosynthesis and respiration, and allows us to partition respiratory sources between soil, stems and leaves. Presently, my work has already shown soils from different regions vary in the rapidity of oxygen isotopic exchange, but there is great uncertainty how different soils, and leaf water isotopic signatures in different climatic situations interact to provide the signal discernable in bulk atmospheric CO2. In this proposal I will define more precisely how the oxygen of water and CO2 changes temporally and spatially, so as to improve regional predictive modelling of the interactions between soil and biomass respiration under different climatic situations. Ultimately, we must understand the balance between photosynthesis and respiration in terrestrial ecosystems, and especially forests, if we are to develop earth system policies for continued carbon sequestration.