In agricultural landscapes, variation and ecological plasticity in depth of water uptake by annual and perennial plants is an important means by which vegetation controls hydrological balance. However, little is known about how annual and perennial plants growing in agriculturally dominated landscapes in temperate humid regions vary in their water uptake dynamics. The primary objective of this study was to quantify the depth of water uptake by dominant plant species and functional groups growing in contrasting annual and perennial systems in an agricultural landscape in Central Iowa. We used stable oxygen isotope techniques to determine isotopic signatures of soil water and plant tissue to infer depth of water uptake at five sampling times over the course of an entire growing season. Our results suggest that herbaceous species (Zea mays L., Glycine max L. Merr., Carex sp., Andropogon gerardii Vitman.) utilized water predominantly from the upper 20 cm of the soil profile and exhibited a relatively low range of ecological plasticity for depth of water uptake. In contrast, the woody shrub (Symphoricarpos orbiculatus Moench.) and tree (Quercus alba L.) progressively increased their depth of water uptake during the growing season as water became less available, and showed a high degree of responsiveness of water uptake depth to changes in precipitation patterns. Coexisting shrubs and trees in the woodland and savanna sites extracted water from different depths in the soil profile, indicating complementarity in water uptake patterns. We suggest that deep water uptake by perennial plants growing in landscapes dominated by rowcrop agriculture can enhance hydrologic functioning. However, because the high degree of ecological plasticity allows some deep-rooted species to extract water from surface horizons when it is available, positive effects of deep water uptake may vary depending on species' growth patterns and water uptake dynamics. Knowledge about individual species' and plant communities' depth of water uptake patterns in relation to local climate conditions and landscape positions can provide valuable information for strategically incorporating perennial plants into agricultural landscapes to enhance hydrologic regulation.
