Respiration, broadly defined, is any metabolic process in which absorbed oxygen is combined with carbon in an organism to form carbon dioxide (CO2) and generate energy. In soils, there are two main sources of respired CO2:

• The decomposition of organic matter by soil microbes (heterotrophic respiration, or Rh) and;
• the respiration of roots including the respiration of microbes in the rhizosphere, of micorrhizae and of symbiotic microbes, which are all fed by root exudates (autotrophic respiration, or Ra).

Soil respiration can be written in equation form as the sum of these two sources:

Globally soils contain about 3200 Pg carbon (C), which is 2.5 times greater than that contained in the atmosphere and land plants together, and Rs, estimated to release ~78 Pg C annually to the atmosphere, is an order of magnitude greater than the CO2 released in fossil fuel emissions. Thus Rs is a key component of the global C cycle and its measurement is required to improve our understanding of the mechanisms of soil CO2 production and the observed responses of soil CO2 efflux to changes in environmental variables (e.g. soil temperature and moisture, soil nutrient content).

The non-steady state chamber method is an important tool for measuring soil respiration. The main principle behind this method is to measure the rate of change in CO2 concentration in the chamber headspace following closure of the chamber lid. If this measurement is done over a short enough time interval that CO2 concentrations rise linearly and the environmental conditions inside the chamber (e.g. air temperature, humidity, pressure) do not differ appreciably from the ambient environmental conditions outside the chamber, the measured soil CO2 efflux is an accurate representation of the actual soil CO2 efflux.

The soil CO2 efflux, F (in µmol m-2 s-1), can be written as:

where: