Cumulative response of ecosystem carbon and nitrogen stocks to chronic CO(2) exposure in a subtropical oak woodland

SUMMARY: Rising atmospheric carbon dioxide (CO(2)) could alter the carbon (C) and nitrogen (N) content of ecosystems, yet the magnitude of these effects are not well known. We examined C and N budgets of a subtropical woodland after 11 yr of exposure to elevated CO(2). We used open-top chambers to manipulate CO(2) during regrowth after fire, and measured C, N and tracer (15)N in ecosystem components throughout the experiment. Elevated CO(2) increased plant C and tended to increase plant N but did not significantly increase whole-system C or N. Elevated CO(2) increased soil microbial activity and labile soil C, but more slowly cycling soil C pools tended to decline. Recovery of a long-term (15)N tracer indicated that CO(2) exposure increased N losses and altered N distribution, with no effect on N inputs. Increased plant C accrual was accompanied by higher soil microbial activity and increased C losses from soil, yielding no statistically detectable effect of elevated CO(2) on net ecosystem C uptake. These findings challenge the treatment of terrestrial ecosystems responses to elevated CO(2) in current biogeochemical models, where the effect of elevated CO(2) on ecosystem C balance is described as enhanced photosynthesis and plant growth with decomposition as a first-order response.