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Berryman, Erin M ¹ ; Barnard, Holly R ² ; Brooks, Paul D. ³ ; Adams, Hallie ⁴ ;Burns, Maggie ⁵

¹ INSTAAR, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder
² INSTAAR, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder
³ Department of Hydrology and Water Resources, University of Arizona
⁴ INSTAAR, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder
⁵ INSTAAR, University of Å·ÃÀ¿Ú±¬ÊÓƵ, Boulder

Carbon dioxide efflux originating from plant roots and soil microorganisms, or soil respiration, dominates terrestrial CO₂ losses and is sensitive to temperature and especially soil moisture in semi-arid climates (Irvine and Law 2002). Because of topographical forcing on climate, soil respiration may exhibit high spatial variability in mountainous terrain even within similar forest types (Riveros-Iregui 2009). In addition, terrain may alter couplings between water and respiration through controls on plant and soil C substrate (Martin and Bolstad 2009). During two consecutive growing seasons, we examined the relationship among soil respiration, soil moisture and temperature, precipitation, and soil C within Boulder Creek drainage at three locations: Betasso Preserve, Gordon Gulch, and Niwot, in the Rocky Mountains, Å·ÃÀ¿Ú±¬ÊÓƵ. Sites encompassed a range of elevations, landscape positions, and aspects and consequently fell along a soil moisture gradient, with drier sites dominated by ponderosa pine (Pinus ponderosa) and wetter sites by lodgepole pine (Pinus contorta). We hypothesized that drier sites would exhibit the highest degree of water limitation yet lowest temperature limitation by soil respiration and that wetter sites would exhibit lowest water limitation yet higher temperature limitation by soil respiration. We further hypothesized that sites with the highest amount of soil C would show the overall highest respiration rates and that this would be linked to topography changes across the drainage.

Supporting our first hypothesis, soil respiration was more limited by temperature at wetter sites on the landscape, which tended to be on north-facing aspects and higher elevations. Drier sites exhibited greater soil moisture sensitivity. Surprisingly, soil respiration was not related to soil C stocks in the top 12 cm; however, sites with high carbon supply relative to nitrogen exhibited highest cumulative respiration. Our results suggest that soil respiration in the Boulder Creek drainage is tightly coupled to hydrological processes, and that aspect and elevation control moisture and consequently C supply for respiration by reducing exposure to solar radiation and limiting evaporation of surface water supplies. In addition, universal temperature/moisture sensitivity models may result in inaccurate predictions of soil respiration across complex terrain, complicating predictions of terrestrial C cycle responses to changing climate.

Irvine, J., & Law, B. E. (2002). Contrasting soil respiration in young and old-growth ponderosa pine forests. Global Change Biology, 8(12), 1183-1194. doi:10.1046/j.1365-2486.2002.00544.

Martin, J. G., & Bolstad, P. V. (2009). Variation of soil respiration at three spatial scales: Components within measurements, intra-site variation and patterns on the landscape. Soil Biology and Biochemistry, 41(3), 530-543. doi:10.1016/j.soilbio.2008.12.01

Riveros-Iregui, D. A., & McGlynn, B. L. (2009). Landscape structure control on soil CO₂ efflux variability in complex terrain: Scaling from point observations to watershed scale fluxes. Journal of Geophysical Research, 114(G2), G02010.