What was done
This complex study was designed to determine if elevated CO2 concentrations alter litter decomposition rates in California grasslands by changing litter quality of individual species, carbon allocation patterns of individual species, species composition in ecosystems, patterns of soil moisture, and microbial community size and composition.  Thus, the authors placed bags filled with varying proportions of mixed-species litter, produced under ambient and twice-ambient atmospheric CO2 concentrations, in the ground, on the soil surface, and suspended above the ground in open-top chambers receiving 350 and 700 ppm CO2 for eight months prior to assessing decomposition rates.

What was learned
Even though mixed-species litter containing greater proportions of litter from nitrogen-fixing leguminous species possessed greater initial nitrogen contents than other mixed-species litter, litter nitrogen content (litter quality) had little to no influence on decomposition rates, as did soil moisture, unmeasured microbial activities, and atmospheric CO2 concentrations during growth and decomposition.  Thus, the most significant differences in decomposition were not caused by atmospheric CO2 enrichment, but rather by litter position in the open-top chambers, wherein suspended litter samples decomposed slower than surface samples, which in turn decomposed slower than buried litter samples.  Such observations led the authors to state that "although the rise in atmospheric [CO2] has the potential to alter litter decomposition rates through many mechanisms, our results indicate that few of these mechanisms will drive biologically significant changes."

With respect to nitrogen cycling, litter with higher initial nitrogen contents, as was the case for mixed-species litter containing high proportions of litter from nitrogen-fixing legumes, released more nitrogen during decomposition than litter containing lower initial nitrogen contents, regardless of atmospheric CO2 concentration.  Thus, if nitrogen-fixing species increase their dominance in ecosystems in response to rising CO2 levels, which has been reported to occur, the authors suggest that "any increase in (soil) nitrogen availability could relieve hypothesized increases in nitrogen limitation under elevated [CO2]."  In other words, plants would likely exhibit even greater productivity and growth responses to rising CO2 levels if greater amounts of nitrogen are being released into soils following the decomposition of nitrogen-fixing species.

What it means
The results of this study suggest that as the CO2 content of the air continues to rise, it will have few significant or important effects on litter decomposition in California grasslands.  However, increasing atmospheric CO2 concentrations may increase nitrogen cycling in such ecosystems, especially if nitrogen-fixing leguminous species increase in abundance in response to elevated CO2.  This latter phenomenon then could well increase the absolute magnitude of plant growth responses to atmospheric CO2 enrichment.