ELEVATED CO2 AND MOISTURE EFFECTS ON SOIL CARBON STORAGE AND CYCLING IN TEMPERATE GRASSLANDS

In grassland ecosystems, most of the carbon (C) occurs below-ground. U nderstanding changes in soil fluxes induced by elevated atmospheric CO 2 is critical for balancing the global C budget and for managing grass land ecosystems sustainably. In this review, we use the results of sho rt-term (1-2 years) studies of below-ground processes in grassland com munities under elevated CO2 to assess future prospects for longer-term increases in soil C storage. Results are broadly consistent with thos e from other plant communities and include: increases in below-ground net primary productivity and an increase in soil C cycling rate, chang es in soil faunal community, and generally no increase in soil C stora ge. Based on other experimental data, future C storage could be favour ed in soils of moderate nutrient status, moderate-to-high clay content , and low (or moderately high) soil moisture status. Some support for these suggestions is provided by preliminary results from direct measu rements of soil C concentrations near a New Zealand natural CO2-ventin g spring, and by simulations of future changes in grassland soils unde r the combined effects of CO2 fertilization and regional climate chang e. Early detection of any increase in soil C storage appears unlikely in complex grassland communities because of (a) the difficulty of sepa rating an elevated CO2 effect from the effects of soil factors includi ng moisture status, (b) the high spatial variability of soil C and (c) the effects of global warming. Several research imperatives are ident ified for reducing the uncertainties in the effects of elevated atmosp heric CO2 on soil C.