Rising atmospheric CO₂ concentrations increase the plant demand for soil nutrients, which in turn can impose a nitrogen limitation on unmanaged ecosystems. The microbial responses to CO₂ enrichment are complex and difficult to predict. Some studies suggest that CO₂ enrichment increases microbial mineralization of nitrogen, making nitrogen more available through a carbon priming effect. Alternatively, microbes may contribute to nitrogen limitation through accelerated soil nitrogen losses. In this study, we examined the effects of CO₂ enrichment on trace gases that are released or taken up during soil microbial reactions: nitrification, denitrification and methane consumption. Ambient and approximately twice-ambient CO₂ treatments were applied to a coastal scrub oak community at Kennedy Space Center, FL, via open-top chambers since May 1996. The CO₂ treatments ended in July 2007 before an aboveground harvest took place inside the chambers. Nitrous oxide (N₂O), nitric oxide (NO) and methane (CH₄) fluxes were measured in the field from 2006-2008.
Results/Conclusions

Soil N₂O losses from the study site were low (< 1 ng N₂O-N cm^-2 h^-1) with no CO₂ treatment effect. Soil NO losses were similarly low (< 1 ng N₂O-N cm^-2 h^-1), but fluxes were consistently lower in elevated CO₂ than in ambient CO₂. NO production was higher for 3 months post-harvest in ambient CO₂. Methane consumption was lower in elevated vs. ambient CO₂ in 2006, although this trend was not significant. Over a decade of CO₂ enrichment has reduced soil nitrogen availability, which could explain the low overall rates of nitrogen trace gas emission. Reduced soil carbon stores in elevated CO₂ measured at this site could also explain the lower nitrification rates, measured as NO efflux. Trace gas emissions in this sandy, nutrient-poor scrub oak forest are comparable to published rates in desert ecosystems.