Spatiotemporal Variability of Soil Water δ18O and δ2H Reveals Hydrological Processes in Two Floodplain Soils

ABSTRACT Soil water budgets in floodplains are distinct from uplands because there are more potential sources of water yet remain poorly understood and poorly represented in the empirical literature. Stable isotopes of hydrogen (2H) and oxygen (18O) in water are useful as tracers of water movement and have improved conceptual understanding of soil hydrological processes. We sampled two adjacent microsites at a ridge‐swale sequence in a forested floodplain Louisiana, USA to determine temporal and spatial soil water isotopic variability in soils of similar climate and ecosystem but contrasting textures and hydrological processes. Repeated soil borings indicated that soil water isotopic variability was greater than any source water sampled (range of δ18O (‰): −11.9 to −0.9, range of δ2H (‰): −44 to 0; n = 783) including throughfall (range of δ18O (‰): −7.6 to +4.7, range of δ2H (‰): −40 to +11; n = 162), groundwater (range of δ18O (‰): −6.2 to 0, range of δ2H (‰): −31 to −7; n = 39), ponded water within the swale (range of δ18O (‰): −6.3 to +0.2, range of δ2H (‰): −33 to −4; n = 16), and free water from boreholes (range of δ18O (‰): −21 to +5, range of δ2H (‰): −4.5 to +5.8; n = 64). Soil water isotopic variability among samples of the same depth and date was not related to soil texture, organic content, or water content, but was instead likely related to seasonal wet up. Soil water isotopic mixing models indicated soil water recharge regimes varied by microsite: Swale soils were dominated by replacement of event water and ridge soils were dominated by conservative mixing at the profile scale. The isotopic composition of bound water was less variable spatially within structured clay soil than it was in coarser textured soil (average difference in s2 0.241 for δ18O and 2.6 for δ2H) apparently because the microtopography of the site allowed for greater inundation and more persistent source waters among swale soils. Soil borings indicated that bound water isotopic variability was greater than any mobile source water sampled. The isotopic composition of bound water was less spatially variable within structured clay soil at the swale microsite than it was in coarser textured soil at the adjacent ridge microsite. There was no strong relationship between bound water isotopic variability and soil water content or organic matter. Findings suggest the microtopography of the study area was likely an influence on bound water variability.