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Title: Microbially Mediated CH4 Consumption and N2O Emission is Affected by Elevated CO2, Soil Water Content, and Composition of Semi-Arid Grassland Species

Author
item Dijkstra, Feike
item Morgan, Jack
item Lecain, Daniel
item Follett, Ronald

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2009
Publication Date: 3/15/2010
Citation: Dijkstra, F.A., Morgan, J.A., Lecain, D.R., Follett, R.F. 2010. Microbially Mediated CH4 Consumption and N2O Emission is Affected by Elevated CO2, Soil Water Content, and Composition of Semi-Arid Grassland Species. Plant and Soil Journal. 329:269-281.

Interpretive Summary: The global atmospheric CO2 concentration continues to rise causing large impacts on terrestrial ecosystems. Semi-arid grasslands in particular are vulnerable to rising CO2 concentrations affecting plant species composition and soil water availability. In return, these changes in plant species composition and soil water availability affect carbon and nutrient cycling. A big unknown is how elevated CO2 and its effects on plant species composition and soil water availability will alter the consumption/emission of the greenhouse gasses methane (CH4) and nitrous oxide (N2O). A greenhouse experiment was conducted to better understand the relative effects of elevated CO2, soil water availability, and composition of semi-arid grassland species on CH4 consumption and N2O emission. Elevated CO2 increased CH4 consumption by 37%, while soil water availability and plant species composition did not significantly affect CH4 consumption. On the other hand, soil water availability and plant species composition had a significant effect on N2O emission, while elevated CO2 had no significant effect. These results indicate that to better predict the effects of elevated CO2 on CH4 and N2O fluxes and to develop management practices to minimize greenhouse gas emissions in these grassland systems, effects of elevated CO2 and the CO2-induced changes in soil moisture and species composition on CH4 and N2O fluxes need to be considered.

Technical Abstract: Rising atmospheric CO2 concentration alters C and N cycling in terrestrial ecosystems. These changes in C and N cycling are to a large extent caused by CO2 effects on water availability and plant species composition, but their separate and interactive effects have rarely been tested simultaneously. We studied the effects of atmospheric CO2 concentration (400 vs 780 ppm), water availability (15 vs 20% gravimetric soil moisture), and composition of semi-arid grassland species (perennial grasses Bouteloua gracilis, Pascopyrum smithii, and Stipa comata; sub-shrub Artemisia frigida; invasive forb Linaria dalmatica grown in monoculture and all five species together) on CH4 consumption and N2O emission in a full factorial greenhouse experiment. We used a unique method to measure microbial CH4 consumption where we eliminated effects of CH4 diffusion. Because biological activity in semi-arid grasslands strongly depends on water, we hypothesized that potential CO2, water, and plant species composition effects on CH4 consumption and N2O emission could be associated with the treatment effects on soil moisture. Methane consumption expressed by a consumption rate constant was significantly higher under elevated CO2 (37%), particularly with low water availability (significant CO2*water availability interaction). Methane consumption did not differ among species. Nitrous oxide emission was not significantly affected by elevated CO2, but was significantly higher with high water availability (67%) and differed significantly among species. Treatment effects on CH4 consumption and N2O emission often could not be explained simply by differences in soil moisture, suggesting that other factors played a role in causing these treatment effects.