ABSTRACT:

Brief Summary:
Soil physics simulations showed water isotope ratios can differ among drainage, mobile and immobile storages due to transport processes alone, but effects were smaller than field data implying unrepresented processes underly ecohydrologic separation.

Manuscript Abstract:
Field measurements of hydrologic tracers indicate varying magnitudes of geochemical separation between subsurface pore waters. The potential for conventional soil physics alone to explain isotopic differences between preferential flow and tightly-bound water remains unclear. Here, we explored physical drivers of isotopic separations using 650 different model configurations of soil, climate, and mobile/immobile soil-water domain characteristics, without confounding fractionation or plant uptake effects. We find simulations with coarser soils and less precipitation led to reduced separation between pore spaces and drainage. Amplified separations were found with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Nonetheless, isotopic separations remained small (<4‰ for d2H) across simulations, indicating that contrasting transport dynamics generate limited geochemical differences. Therefore, conventional soil physics alone are unlikely to explain large ecohydrological separations observed elsewhere, and further efforts aimed at reducing methodological artifacts, refining understanding of fractionation processes, and investigating new physiochemical mechanisms are needed.

Dataset Information:
This dataset contains HYDRUS model output used to create four figures in a study by C. Finkenbiner of the ability of soil physics models to represent isotopic separation. Within the four provided folders are model output files with filenames describing their contents. Simulations were ran for 300 days, with the last 100 days used for analysis provided here. Output files are csv files that contain H2 isotope ratios, soil moisture, or soil drainage values across 10 simulations, denoted pv1 - pv10. Simulations were configured to have high, low or zero fractions of immobile soil pore space, denoted as Hf, Lf, and 0f respectively in filenames. Simulations with immobile pore spaces were also configured to have high and low transfer coefficients, denoted as Hw and Lw in file names. For more information please see Finkenbiner et. al. 2022 in Nature Communications (DOI: 10.1038/s41467-022-34215-7) or contact Stephen Good at Oregon State University.

ABSTRACT:

The National Ecological Observatory Network (NEON) provides open-access measurements of stable isotope ratios in atmospheric water vapor (δ2H, δ18O) and carbon dioxide (δ13C) at different tower heights, as well as aggregated biweekly precipitation samples (δ2H, δ18O) across the United States. These measurements were used to create the NEON Daily Isotopic Composition of Environmental Exchanges (NEON-DICEE) dataset estimating precipitation (P; δ2H, δ18O), evapotranspiration (ET; δ2H, δ18O), and net ecosystem exchange (NEE; δ13C) isotope ratios. Statistically downscaled precipitation datasets were generated to be consistent with the estimated covariance between isotope ratios and precipitation amounts at daily time scales. Isotope ratios in ET and NEE fluxes were estimated using a mixing-model approach with calibrated NEON tower measurements. NEON-DICEE is publicly available on HydroShare and can be reproduced or modified to fit user specific applications or include additional NEON data records as they become available. The NEON-DICEE dataset can facilitate understanding of terrestrial ecosystem processes through their incorporation into environmental investigations that require daily δ2H, δ18O, and δ13C flux data.