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<span class="paraNumber"></span> We reviewed responses of nitrification, denitrification, and soil N<sub>2</sub>O efflux to elevated CO<sub>2</sub>, N availability, and temperature, based on published experimental results. We used meta-analysis to estimate the magnitude of response of soil N<sub>2</sub>O emissions, nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and net and gross nitrification across experiments. We found no significant overall effect of elevated CO<sub>2</sub> on N<sub>2</sub>O fluxes. DEA and NEA significantly decreased at elevated CO<sub>2</sub>; however, gross nitrification was not modified by elevated CO<sub>2</sub>, and net nitrification increased. The negative overall response of DEA to elevated CO<sub>2</sub> was associated with decreased soil [NO<sub>3</sub><sup>−</sup>], suggesting that reduced availability of electron acceptors may dominate the responses of denitrification to elevated CO<sub>2</sub>. N addition significantly increased field and laboratory N<sub>2</sub>O emissions, together with gross and net nitrification, but the effect of N addition on field N<sub>2</sub>O 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 CO<sub>2</sub> 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 CO<sub>2</sub>.
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Atmospheric CO<sub>2</sub> 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 CO<sub>2</sub> is leaf litter decomposition. We investigated the interactions among atmospheric CO<sub>2</sub>, 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 CO<sub>2</sub>, and eight were exposed to elevated levels of CO<sub>2</sub> (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 CO<sub>2</sub>. 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 CO<sub>2</sub> concentration. However, changes in litter chemistry from year to year were far larger than effects of CO<sub>2</sub> or insect damage, suggesting that these may have only minor effects on litter decomposition.
Menyailo OV, Hungate BA (2005) Tree species effects on potential production and consumption of carbon dioxide, methane, and nitrous oxide: the Siberian afforestation experiment. Tree Species Effects on Soils: Implications for Global Change 293-305.Read Abstract / Download .PDF / Read Publication
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 CO<sub>2</sub> production, N<sub>2</sub>O-reduction and N<sub>2</sub>O production during denitrification, and CH<sub>4</sub> 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 N<sub>2</sub>O and CH<sub>4</sub> fluxes than on CO<sub>2</sub> 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 N<sub>2</sub>O reduction were idiosyncratic, resulting in a decoupling of N<sub>2</sub>O production and reduction. CH<sub>4</sub> oxidation was affected by tree species, but these effects depended on soil moisture: increasing soil moisture enhanced CH<sub>4</sub> 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 CH<sub>4</sub> but also to produce more N<sub>2</sub>O. Future predictions of CH<sub>4</sub> uptake and N<sub>2</sub>O efflux in boreal and temperate forests need to consider changes in tree species composition together with changes in soil moisture regimes.