Variation in water uptake dynamics among contrasting agricultural and native plant communities in the Midwestern US

Landscape-scale conversion of deeply rooted perennial native vegetation to annual crops has altered the ecohydrological balance in the Midwestern U.S. One of the major ways this may occur is through plant species' abilities to access and take up water from different depths in the soil profile. In this study, we use stable isotope techniques to assess plant water uptake patterns for plants growing in four contrasting annual and perennial ecosystems in central Iowa: row-cropped cornfield, prairie, oak savanna, and woodland. To address limitations of conventional approaches to analyzing isotopic data, especially when isotopic profiles in the soil are complex, we employ an alternative,multiple source mass-balance isotopic' approach suitable for determining probable contributions of multiple potential sources to total plant water uptake. Observed differences in isotopic profiles among the plant communities were attributed to differences in infiltration rates, plant water uptake rates, the degree of evaporative isotopic fractionation under the different vegetative covers, and processes of hydraulic redistribution. Savanna (Quercus macrocarpa Mixch) and woodland (Q. macrocarpa, Umus americana L.) trees had the lowest stem water delta O-18 values (-6.12 parts per thousand, -7.77 parts per thousand, and -7.61 parts per thousand, respectively), whereas corn (Zea mays L.) and prairie (Andropogon gerardii) exhibited higher stem water delta O-18 values (-5.49 parts per thousand and -5.85 parts per thousand, respectively), suggesting more shallow sources of water uptake for the latter. Frequency histographs produced from the multiple source mass-balance analysis confirmed that the corn and prairie species obtained up to 45% and 36% of their water from the upper 0-20 cm soil horizon, respectively. In contrast, oak trees growing in the savanna and woodland obtained up to 40% and 20% of their water from the upper 20 cm, and up to 60% and 80% of their water from depths > 60 cm, respectively. Lack of strong isotopic gradients in the soil profiles combined with irregularities in the slope of the isotopic gradient by depth complicated interpretation of the data. However, an important advantage of the alternative multiple source mass-balance isotopic approach over conventional approaches was its capability of producing more quantitative and objective estimates for the probable contributions of multiple sources to total plant water uptake. This study highlights how changes in land use and vegetative cover in the Midwest affects plant water uptake patterns, and suggests how these changes may influence larger landscape scale alterations in the hydrologic balance. (c) 2006 Elsevier B.V. All rights reserved.