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

Authors

Theresa Marie Lorenzo, University of Nebraska-LincolnFollow

Date of this Version

5-2015

Document Type

Article

Citation

Lorenzo, T. E. Hydrologic Mediation of the Spatial and Temporal Variability of the Soil Carbon Dioxide Stable Isotopic Composition of a Subalpine Watershed. MS Thesis, University of Nebraska-Lincoln. 2015.

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Natural Resource Sciences, Under the Supervision of Professor Diego Riveros-Iregui. Lincoln, Nebraska: May, 2015

Copyright (c) 2015 Theresa Marie E. Lorenzo

Abstract

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-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 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-Iregui