A Priming Effect of Elevated CO2 on
Soil Organic Matter Decomposition

There has been very little recognition in elevated CO2 studies of the fact that carbon flow into fast-cycling pools can influence long-term carbon storage. For example, elevated CO2 retarded decomposition of the slow-cycling soil organic carbon pool in a California grassland (Cardon et al., 2001). This apparently occurred because soil microorganisms shifted from consuming older, presumably more recalcitrant, soil organic carbon pools to using easily-degraded roots and root exudates derived from increased root biomass.

Effect of plants and elevated CO2
on decomposition of native soil organic matter.


Soil w/ Plants

Soil w/o Plants


Plant respiration

soil respiration

soil respiration









We performed a greenhouse study to investigate the influence of elevated CO2 on the decomposition of the native soil organic pool. We grew Scirpus olneyi that was depleted in 13 C (d 13 C = -27 o / oo to -39 o / oo ) in a wetland soil that was relatively enriched in 13 C (d 13 C= -15 o / oo ). This allowed us to partition soil respiration into plant and soil sources using a standard 2-end member mixing model (Fu and Cheng, 2002).

The results showed a priming effect of plant roots on soil organic matter decomposition because soil respiration was about 7-times higher in soils that contained plants (Table 4). Elevated CO2 increased soil respiration by 22%, which is similar to the average stimulation of CO2 assimilation by elevated CO2 in situ . This result should be considered preliminary. Nonetheless, we conclude that elevated CO2 can stimulate the decomposition of native soil organic matter in this salt marsh ecosystem. We currently have a field test of this hypothesis running in which we buried membrane bags containing soil organic matter in each C3 plot. These bags will be collected next fall and analyzed for mass loss and changes in the content of labile and recalcitrant carbon.


Research Questions