Soil respiration and the global carbon cycle

Soil respiration is the primary path by which CO2 fixed by land plants retu rns to the atmosphere. Estimated at approximately 75 x 10(15) gC/yr, this l arge natural flux is likely to increase due changes in the Earth's conditio n. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contrib ute to the rise in CO2 in Earth's atmosphere. Rising concentrations of CO2 in the atmosphere will increase the flux of CO2 from soils, while simultane ously leaving a greater store of carbon in the soil. Traditional tillage cu ltivation and rising temperature increase the flux of CO2 from soils withou t increasing the stock of soil organic matter. Increasing deposition of nit rogen from the atmosphere may lead to the sequestration of carbon in vegeta tion and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric CO2 during the nex t century.