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Title: The relative controls of temperature, soil moisture, and plant functional group on soil CO2 efflux at diel, seasonal, and annual scales

item BARRON-GAFFORD, G.A. - University Of Arizona
item Scott, Russell - Russ
item JENERETTE, G.D. - University Of Arizona
item HUXMAN, T.E. - University Of Arizona

Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2011
Publication Date: 3/8/2011
Citation: Barron-Gafford, G., Scott, R.L., Jenerette, G., Huxman, T. 2011. The relative controls of temperature, soil moisture, and plant functional group on soil CO2 efflux at diel, seasonal, and annual scales. Journal of Geophysical Research. 116:1-16.

Interpretive Summary: A major challenge in quantifying carbon dioxide exchange dynamics within ecosystems lies in identifying whether landscapes are sources or sinks for atmospheric carbon across seasonal, annual, and decadal timescales. Because soil respiration is such a dominant contributor to overall ecosystem efflux, it is important to quantify how temperature, moisture, and plant activity influences respiration. We used automated measurement systems to quantify soil respiration across an entire year in three main microhabitats (under mesquite trees, under bunchgrasses, and in intercanopy soil spaces) of a semiarid grassland and relate patterns of respiration to environmental conditions. We found that microhabitat dramatically influenced the results with respiration under the mesquite trees much larger than under grasses or bare soil. We also found that respiration was not effected by temperature in the way predicted by a commonly-used theory. As mesquite plants in semiarid regions continue to expand into former grasslands, the increased carbon uptake by the more productive ecosystems commonly found in these ecosystems may be partially negated or even entirely offset by the increased carbon losses from the soil.

Technical Abstract: Soil respiration (Rsoil) is a dominant, but variable, contributor to ecosystem CO2 efflux. Understanding how variations in major environmental drivers, like temperature and available moisture, might regulate Rsoil has become extremely relevant. Plant functional-type diversity makes such assessments difficult because of the confounding influence of varied plant phenology and influences on soil microhabitats. We used automated measurement systems to quantify Rsoil in the three microhabitats (under mesquites, under bunchgrasses, and in intercanopy soils) that result from mesquite encroachment into grasslands to inform our understanding of diel Rsoil patterns in response to changes in temperature, seasonal variations in Rsoil in response to varied soil moisture and plant phenology, and the contribution of each microhabitat to total ecosystem-scale Rsoil. We detected a counterclockwise hysteretic response of Rsoil to soil temperature, such that up to 100% greater fluxes were observed in the afternoon/evening than the morning for the same temperature. Phenological differences influenced ecosystem-scale Rsoil in that mesquites were physiologically active months before bunchgrasses and Rsoil rates under mesquites were greater and elevated longer in response to rains. Cumulative annual Rsoil was 412, 229, and 202 g C m-2 under mesquites, bunchgrasses, and intercanopy spaces, respectively. Extrapolating to the ecosystem-scale using cover estimates within the site’s eddy covariance footprint illustrated that average mesquite Rsoil contributed 46% to overall ecosystem-scale Rsoil, though mesquite composed only about 35% of the site. As grasslands transition to shrub dominance, the contribution of Rsoil to net ecosystem flux will likely increase, potentially offsetting presumed greater CO2 uptake potential of woody plants.