Influences of vegetation coverage and surface microtopography on spatiotemporal variation of soil water isotopic composition

Abstract:

Measuring soil water isotopes has helped to address questions of water movement through the soil-plant-atmosphere continuum. Many studies have focused on how soil water isotope signatures change over time in relation to nearby vegetation, but recent research has highlighted that spatial variation, both vertical and lateral, of bulk soil water is important to consider when identifying plant water sources. However, other studies have shown via stable water isotopes that bulk soil water is not always well mixed because highly mobile soil water from larger pores can be isotopically distinct from less mobile soil water from small pores and bound to soil particles surfaces. We set out to determine if the spatial variation of highly mobile soil water and less mobile soil water is necessary to consider and if it correlates to spatial vegetation coverage and surface microtopography. We set up a 50 meter square plot on a south facing hillslope in a mixed conifer forest in southeastern Wyoming. Forty soil sample points were randomly selected within the plot and soil samples were taken at two depths, 5-15 cm and 20-30 cm. Soil water was extracted via two sequential methods, centrifugation and cryogenic vacuum distillation, to collect a highly mobile soil water sample and a less mobile soil water samples. In order to determine role of vegetation, 40 random conifers (about 13 of each species: Abies laziocarpa, Picea engelmannia, and Pinus cortata) were cored from each cardinal direction to have a composite sample for each individual. The composite tree samples were extracted via cryogenic vacuum distillation. Sampling occurred roughly a month after snowmelt, late June 2020, with all trees and soil points sampled on two sampling dates about a week apart. The role of microtopography and canopy coverage was assessed with drone imagery used in conjunction with a digital elevation model. We predicted that topographic wetness index and canopy coverage would correlate with spatial variation of soil water isotopes in both the highly mobile soil water pool and less mobile soil water pool. Understanding how canopy coverage and surface microtopography potentially impact spatial variation of soil water isotopes in multiple soil pore domains will greatly benefit the fields of soil hydrology, ecohydrology, and paleoclimatology.