CONTRASTING EFFECTS OF ELEVATED CO2 ON OLD AND NEW SOIL CARBON POOLS

Authors: CARDON, ZOE - UNIVERSITY OF CONNECTICUT; HUNGATE, BRUCE - NORTHERN AZ UNIVERSITY; Cambardella, Cynthia; CHAPPIN III, F - UNIVERSITY OF ALASKA; FIELD, CHRISTOPHER - CARNEGIE INSTITUTE; HOLLAND, ELISABETH - NCAR BOULDER, CO; MOONEY, H - STANFORD UNIVERSITY

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2000
Publication Date: 3/1/2001
Citation: Cardon, Z.G., Hungate, B.A., Cambardella, C.A., Chappin Iii, F.S., Field, C.B., Holland, E.A., Mooney, H.A. 2001. Contrasting effects of elevated co2 on old and new soil carbon pools. Soil Biology and Biochemistry. 33:365-373.

Interpretive Summary: Human activities have altered the global C cycle in the past century, primarily by increasing atmospheric CO2 as a result of land use changes and the burning of fossil fuels. Although the role of soil and soil organic matter (SOM) in the global C cycle has been researched extensively for many years, critical gaps in our knowledge about the mechanisms responsible for cycling and partitioning of photosynthetically fixed C into the various forms of soil C still exits. Notably, there is a lack of information on belowground C allocation and cycling. This study examined the impact of elevated atmospheric CO2 on C cycling in two annual grassland ecosystems growing in open-top chambers under natural rainfall and light from 1994-1996. The study clearly demonstrated that increasing the atmospheric CO2 concentration resulted in more plant biomass, but elevated CO2 also altered soil C processing and turnover. The decay rate of older forms of soil C decreased and inputs of new C into roots increased. The combination of these two factors could limit N availability to the plants and ultimately limit plant growth. This research provides scientists and policy makers with new information about the impact of elevated CO2 on soil C dynamics and its subsequent effects on CO2 fluxes between terrestrial ecosystems and the atmosphere.

Technical Abstract: Soil organic carbon (SOC) is the largest reservoir of organic carbon in the terrestrial biosphere. Increasing atmospheric CO2 may have important indirect effects on the breakdown of large, preexisting SOC pools. This study examines how elevated and ambient CO2 impacted C inputs and outputs from two annual grassland ecosystems growing in open-top mesocosms at low and high soil nutrient availability under natural rainfall and light from 1994-1996. To distinguish newly-fixed C from older soil C, we grew two C3 plant communities in soil obtained from a northeastern Colorado C4 grassland. We utilized isotopic methods to determine the size and origin of soil C pools and the isotopic composition of soil atmospheric CO2 to distinguish the breakdown of older C4 C in soil organic matter from the breakdown of newly-fixed C3 C. We found that the absolute amount of newly fixed C invested belowground in new root biomass increased under elevated CO2, but the movement of newly-fixed C from roots into stabilized, mineral-bound pools was retarded. Elevated CO2 also retarded the decomposition of older SOC when mineral nutrients were abundant, thus increasing the turnover time of SOC already in the system. Under elevated CO2, soil microorganisms appeared to shift from consuming older SOC to utilizing easily-degraded rhizodeposits derived from increased root biomass. The resulting buildup of root litter with high C:N ratio, combined with decreased decay of older SOC, could limit N availability to plants and eventually decrease net primary productivity. These contrasting effects of elevated CO2 on soil C pool dynamics contribute to a new soil C equilibrium that could profoundly affect long-term net C movement between terrestrial ecosystems and the atmosphere.