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 CO2 (δ13C-CO2) has been studied as an indicator of changes in ecosystem CO2 exchange. Soil moisture is an important factor in ecosystem CO2 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 δ13C-CO2 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 (δ13C) and concentrations of soil CO2 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 13C-CO2 varies systematically with topography and soil moisture gradients. In response to a soil moisture drydown, bulk soil δ 13C-CO2 and the calculated δ13C-CO2 value of the biological sourcein upland areas became more positive. This is consistent with an increase in plant δ13C-CO2 with drought stress due to a decrease in photosynthetic discrimination, and has been observed previously at smaller scales. In contrast, soil d13C-CO2 did not change significantly in riparian areas, where soil moisture remained high throughout the field season. Elevation was positively correlated with soil d13C-CO2, 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 d13C-CO2 during the same time period. Plot soil moisture measurements were better predictors of soil CO2 concentration and soil CO2 flux than topographical attributes. This study indicates that in complex terrain at high to medium soil moisture levels, the variability of soil δ13C-CO2 is systematically linked to landscape position, possibly largely due to the influence of topographical heterogeneity on soil moisture distribution. Adviser: Diego Riveros-Iregu