Questions persist about interpreting isotope ratios of bound and mobile soil water pools, particularly relative to clay content and extraction conditions. Interactions between pools and resulting extracted water isotope composition are presumably related to soil texture, yet few studies have manipulated the bound pool to understand its influence on soil water processes. Using a series of drying and spiking experiments, we effectively labelled bound and mobile water pools in soils with varying clay content. Soils were first vacuum dried to remove residual water, which was then replaced with heavy isotope-enriched water prior to oven drying and spiking with heavy isotope-depleted water. Water was extracted via centrifugation or cryogenic vacuum distillation (at four temperatures) and analysed for oxygen and hydrogen isotope ratios via isotope ratio mass spectrometry. Water from centrifuged samples fell along a mixing line between the two added waters but was more enriched in heavy isotopes than the depleted label, demonstrating that despite oven drying, a residual pool remains and mixes with the mobile water. Soils with higher clay + silt content appeared to have a larger bound pool. Water from vacuum distillation samples have a significant temperature effect, with high temperature extractions yielding progressively more heavy isotope-enriched values, suggesting that Rayleigh fractionation occurred at low temperatures in the vacuum line. By distinctly labelling bound and mobile soil water pools, we detected interactions between the two that were dependent on soil texture. Although neither extraction method appeared to completely extract the combined bound and mobile (total water) pool, centrifugation and high temperature cryogenic vacuum distillations were comparable for both δ 2 H and δ 18 O of soil water isotope ratios. K E Y W O R D Sclay mineralogy, extraction, methods, soil texture, soil water, stable isotopes, two water world hypothesis

No abstract

No abstract

The Attwater's prairie-chicken (APC; Tympanuchus cupido attwateri) was listed as an endangered species in 1967, and since then multiple studies have addressed ecological and human factors that may have limited their population recovery. In this study, we used stable isotopes of carbon and nitrogen (δ 13 C and δ 15 N) in conjunction with Bayesian mixing models to determine current diets of APCs at the Attwater Prairie Chicken National Wildlife Refuge (APCNWR), and to compare them with past diets derived from the isotopic composition of feathers from museum specimens (1894-1965) to help elucidate potential factors limiting APC recovery. We collected feathers, blood, and feces from APCs at the APCNWR, and feathers from museum specimens. In addition, potential food items (vegetation, spiders, and insects) were collected from APCNWR and analyzed for δ 13 C and δ 15 N. A stable isotope mixing model (MixSIAR) was used to determine diet source contribution to past and present APC populations. δ 13 C values in APC blood were significantly greater in the fall than in the summer. Blood δ 15 N values were significantly greater in the summer than in the winter. δ 13 C values in feathers from museum and contemporary APCs were not different, although δ 13 C values in contemporary feathers were enriched by 0.5‰ relative to museum specimens. Results indicated that insects were the predominant food source in the summer and fall (61-65%) and C 3 vegetation was predominant in the winter (64%). Using isotope values from feathers, the model predicted that the predominant food source during the period of feather growth were insects (62%) followed by C 3 plants (17-18%), spiders (12-18%), and C 4 plants (2-3%). Feathers from contemporary birds had δ 15 N values 0.9‰ lower than those from museum specimens, suggesting potential shifts in diets in APCs currently in the wild relative to the past. We hypothesize that the abundance and species richness of various arthropods may have changed at the APCNWR resulting in current APCs in the wild feeding on arthropods with lower δ 15 N values than in the past. Orthopterans comprised about 23% of the APCs diet currently in the wild, while they represented only about 19% in the past.

No abstract