Ecosystem carbon balance: respiration

Stable isotopes may be used to understand the processes that affect photosynthesis on a whole ecosystem scale. Since plants preferentially use 12CO2 in photosynthesis, the CO2 left behind in the atmosphere is enriched in 13CO2. At the same time, plants and all other organisms in the ecosystem are always respiring, or using oxygen and producing CO2 to maintain metabolic processes.  It is commonly assumed that there is no fractionation during respiration, so the CO2 that is respired from organisms has the same isotope ratio as the live tissue.  Photosynthetic discrimination dominates in the daytime, while respiration dominates at night - this produces a diurnal variation in d13C of ambient CO2.

For C3 plants, respired CO2 is very depleted in 13C, with d on the order of -22 to -35‰.  Because respired CO2 and CO2 left behind in atmosphere after photosynthesis have different isotope ratios, the isotopic composition of CO2 entering and leaving ecosystems can provide information on the balance of photosynthesis and respiration, so that we can better make predictions about how each parameter will change in the future. For instance, respiration is largely regulated by temperature, whereas photosynthesis is affected by light, temperature, drought, and many other factors.

Organisms that live in the soil also respire. Soil organisms range from bacteria and fungi to fauna such earthworms, nematodes, and even mammals. The plant litter that falls on to the soil surface is consumed by many types of organisms, which release CO2 in the process. The chemical components of plant material are not consumed at the same rates, as some are more difficult to digest than others. These chemical substances, which include sugars, lipids, lignin, and cellulose, generally have different isotope ratios within a single plant.  Therefore the carbon isotope ratio of CO2 emitted from the soil can be complex, and may change over time.

Because of the differences in discrimination of 18O during photosynthesis and respiration, the oxygen isotope information contained in CO2 fluxes from ecosystems can be used to separate these two components.  The differences in oxygen discrimination between respiration and photosynthesis are larger than the differences in carbon discrimination, which are generally quite small are require sophisticated instrumentation to detect.  However, d18O can be more temporally and spatially variable than d13C due to environmental influences on precipitation and atmospheric water vapor.  The choice of isotope is dependent on the ecosystem and the question of interest.


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