Hydrologic Mediation of the Spatial and Temporal Variability of the Soil Carbon Dioxide Stable Isotopic Composition of a Subalpine Watershed

The stable carbon isotopic composition of CO 2 (δ 13 C-CO 2) has been studied as an indicator of changes in ecosystem CO 2 exchange. Soil moisture is an important factor in ecosystem CO 2 exchange through its influence on physiological and soil physical processes. However, the majority of previous research analyzing the influence of soil moisture on soil and soil-respired δ 13 C-CO 2 has been conducted with limited consideration of topographical variation, which controls the distribution of soil moisture across a landscape. This study characterized the stable isotopic composition (δ 13 C) and concentrations of soil CO 2 at 5, 20, and 50 cm across seven transects in two subalpine watersheds in the Tenderfoot Creek Experimental Forest, Montana. The results show that soil δ 13 C-CO 2 varies systematically with topography and soil moisture gradients. In response to a soil moisture drydown, bulk soil δ 13 C-CO 2 and the calculated δ 13 C-CO 2 value of the biological source in upland areas became more positive. This is consistent with an increase in plant δ 13 C-CO 2 with drought stress due to a decrease in photosynthetic discrimination, and has been observed previously at smaller scales. In contrast, soil δ 13 C-CO 2 did not change significantly in riparian areas, where soil moisture remained high throughout the field season. Elevation was positively correlated with soil δ 13 C-CO 2, following the negative gradient of soil moisture and atmospheric pressure with increasing elevation. Elevation and soil moisture were significantly correlated for two-thirds of the growing season when soil moisture was at medium-high levels, and elevation was a positive predictor of bulk soil δ 13 C-CO 2 during the same time period. Plot soil moisture measurements were better predictors of soil CO 2 concentration and soil CO 2 flux than topographical attributes. This study indicates that in complex terrain at high to medium soil moisture levels, the variability of soil δ 13 C-CO 2 is systematically linked to landscape position, possibly largely due to the influence of topographical heterogeneity on soil moisture distribution. ACKNOWLEDGEMENTS I am very grateful to my advisor Diego Riveros-Iregui for giving me the opportunity to work with him, for helping me to become a better researcher and scientist, and his guidance and patience in going over concepts, my data, figures, and presentations with me numerous times. I would also like to thank my committee members Terry …