An evaluation of the ecohydrological separation hypothesis in a semiarid catchment

Abstract

The ecohydrological separation hypothesis states that transpiration through plants and drainage to streams and groundwater are sourced from separate soil water pools, which possess distinct isotopic signatures. Evidence for ecohydrological separation has relied on the globally ubiquitous observation that plant water and draining water are isotopically distinct. We evaluated the ecohydrological separation hypothesis in the Dry Creek Experimental Watershed in the semiarid, snow-dominated landscape of southwest Idaho, USA. We found that plant water is indeed isotopically distinct from streams and groundwater. However, we were unable to track those waters to subsurface soil waters, nor were we able to relate soil water mobility to isotopic composition. Soil waters of different mobility can be isotopically similar, and isotopic distinction in soil water can occur for reasons not related to mobility. We suggest that isotopic distinction between rootabsorbed and draining waters may not be an appropriate indicator of ecohydrological separation of soil waters, and that hydrologic explanations for such isotopic distinction may not be sufficient.

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…This fractionation signal in the soil water was of the same magnitude as for surface waters in the peatland drainage network of a raised bog in the Bruntland Burn, where the lc-excess reached values < −5 ‰ and the EL slope ranged from 3.9 to 4.9 between May and September (Sprenger et al, 2017b). In comparison to arid and Mediterranean environments, where SW lc-excess can fall below −20 ‰ (McCutcheon et al, 2016, andreviewed in Sprenger et al, 2016b), the SW lc-excess in the Scottish Highlands remained relatively high. While the lc-excess was usually not significantly different from zero at 15-20 cm soil depth in the studied podzols of the study site, soils studied by McCutcheon et al (2016) in a much drier environment (Dry Creek, Idaho) showed that the SW lc-excess from the surface down to −70 cm was significantly lower than zero.…”

“…In comparison to arid and Mediterranean environments, where SW lc-excess can fall below −20 ‰ (McCutcheon et al, 2016, andreviewed in Sprenger et al, 2016b), the SW lc-excess in the Scottish Highlands remained relatively high. While the lc-excess was usually not significantly different from zero at 15-20 cm soil depth in the studied podzols of the study site, soils studied by McCutcheon et al (2016) in a much drier environment (Dry Creek, Idaho) showed that the SW lc-excess from the surface down to −70 cm was significantly lower than zero. The high soil water storage contributes to the SW lc-excess dynamics shown in the hysteresis pattern for the relationship between SW lc-excess and PET 30 , which revealed that there was a delayed response of the SW lc-excess to the onset and offset of soil evaporation in spring and autumn, respectively.…”

“…While comparisons of the isotopic signal in soil waters with waters of plant tissues have been reported for decades (see review by Ehleringer and Dawson, 1992), recently posed research questions on which water of the soil's pore system is used by plants (Brooks et al, 2010) are enhancing ecohydrological studies on soil-plant interactions in the critical zone. While studies based on two sampling campaigns (Goldsmith et al, 2012;Evaristo et al, 2016) were supportive of the ecohydrological separation, as presented by Brooks et al (2010), newly published work with higher temporal resolution of soil water and xylem water isotope sampling suggests that there are seasonal differences with regard to ecohydrological separation (McCutcheon et al, 2016;Hervé-Fernández et al, 2016). A comparative study by Evaristo et al (2015) showed that the isotopic composition of plant waters -just like soil waters in the upper horizons -are usually kinetically fractionated.…”

“…In line with the call for a higher temporal resolution of soil water isotope sampling (Berry et al, 2017), the highly dynamic isotopic signal during the transition between the dormant and growing seasons underlines the importance of not limiting the soil water isotope sampling to a few sampling campaigns (usually n ≤ 3 in Brooks et al, 2010;Evaristo et al, 2016;Goldsmith et al, 2012), when investigating root water uptake patterns. In the light of recent studies dealing with potential water sources of vegetation that showed how variable the plant water isotopic signal can be over a year (Hervé-Fernández et al, 2016;McCutcheon et al, 2016), a proper understanding of the temporal variability of the potential water sources (i.e., bulk soil water, groundwater, stream water) appears to be critical.…”

Effects of climatic seasonality on the isotopic composition of evaporating soil waters

“…Ehleringer and Dawson, 1992;Dawson et al, 2002;Volkmann et al, 2016;Rothfuss and Javaux, 2017), and identifying plant water sources (e.g. Brooks et al, 2010;Dawson and Ehleringer, 1991;Dawson and Simonin, 2011;Goldsmith et al, 2012;Evaristo et al, 2015;Hervé-Fernández et al, 2016;McCutcheon et al, 2017). A recent review by Sprenger et al (2016) 5 provides an extensive overview of isotope-based studies in the unsaturated zone.…”