Publications

<span class="paraNumber">[1]</span> We reviewed responses of nitrification, denitrification, and soil N₂O efflux to elevated CO₂, N availability, and temperature, based on published experimental results. We used meta-analysis to estimate the magnitude of response of soil N₂O emissions, nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and net and gross nitrification across experiments. We found no significant overall effect of elevated CO₂ on N₂O fluxes. DEA and NEA significantly decreased at elevated CO₂; however, gross nitrification was not modified by elevated CO₂, and net nitrification increased. The negative overall response of DEA to elevated CO₂ was associated with decreased soil [NO₃⁻], suggesting that reduced availability of electron acceptors may dominate the responses of denitrification to elevated CO₂. N addition significantly increased field and laboratory N₂O emissions, together with gross and net nitrification, but the effect of N addition on field N₂O efflux was not correlated to the amount of N added. The effects of elevated temperature on DEA, NEA, and net nitrification were not significant: The small number of studies available stress the need for more warming experiments in the field. While N addition had large effects on measurements of nitrification and denitrification, the effects of elevated CO₂ were less pronounced and more variable, suggesting that increased N deposition is likely to affect belowground N cycling with a magnitude of change that is much larger than that caused by elevated CO₂.

Atmospheric CO₂ concentrations have increased dramatically over the last century and continuing increases are expected to have significant, though currently unpredictable, effects on ecosystems. One important process that may be affected by elevated CO₂ is leaf litter decomposition. We investigated the interactions among atmospheric CO₂, herbivory, and litter quality within a scrub oak community at the Kennedy Space Center, Florida. Leaf litter chemistry in 16 plots of open-top chambers was followed for 3 years; eight were exposed to ambient levels of CO₂, and eight were exposed to elevated levels of CO₂ (ambient + 350 ppmV). We focused on three dominant oak species, <em class="EmphasisTypeItalic ">Quercus geminata</em>, <em class="EmphasisTypeItalic ">Quercus myrtifolia</em>, and <em class="EmphasisTypeItalic ">Quercus chapmanii</em>. Condensed tannin concentrations in oak leaf litter were higher under elevated CO₂. Litter chemistry differed among all plant species except for condensed tannins. Phenolic concentrations were lower, whereas lignin concentrations and lignin/nitrogen ratios were higher in herbivore-damaged litter independent of CO₂ concentration. However, changes in litter chemistry from year to year were far larger than effects of CO₂ or insect damage, suggesting that these may have only minor effects on litter decomposition.

Changes in tree species composition could affect how forests produce and consume greenhouse gases, because the soil microorganisms that carry out these biogeochemical transformations are often sensitive to plant characteristics. We examined the effects of thirty years of stand development under six tree species in Siberian forests (Scots pine, spruce, arolla pine, larch, aspen and birch) on potential rates of soil CO₂ production, N₂O-reduction and N₂O production during denitrification, and CH₄ oxidation. Because many of these activities relate to soil N turnover, we also measured net nitrification and N mineralization. Overall, the effects of tree species were more pronounced on N₂O and CH₄ fluxes than on CO₂ production. Tree species altered substrate-induced respiration (SIR) and basal respiration, but the differences were not as large as those observed for N transformations. Tree species caused similar effects on denitrification potential, net N mineralization, and net nitrification, but effects on N₂O reduction were idiosyncratic, resulting in a decoupling of N₂O production and reduction. CH₄ oxidation was affected by tree species, but these effects depended on soil moisture: increasing soil moisture enhanced CH₄ oxidation under some tree species but decreased it under others. If global warming causes deciduous species to replace coniferous species, our results suggest that Siberian forests would support soil microbial communities with enhanced potential to consume CH₄ but also to produce more N₂O. Future predictions of CH₄ uptake and N₂O efflux in boreal and temperate forests need to consider changes in tree species composition together with changes in soil moisture regimes.