Variation in water uptake dynamics among contrasting agricultural and native plant communities in the Midwestern U.S.

Abstract

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 δ¹⁸O values (−6.12‰, −7.77‰, and −7.61‰, respectively), whereas corn (Zea mays L.) and prairie (Andropogon gerardii) exhibited higher stem water δ¹⁸O values (−5.49‰ and −5.85‰, 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.

Introduction

The cycling of water through the plant–soil–atmosphere continuum in the Midwestern U.S. has been drastically altered as a result of landscape-scale conversion of deeply rooted perennial prairie and savanna ecosystems to more shallow-rooted annual crops of corn and soybean (Andersen et al., 1996, Coppedge et al., 2001). Most remnants of native ecosystems remaining on the landscape today are becoming woodlands due to encroachment by shade-tolerant and fire-sensitive trees (Nuzzo, 1986, Anderson, 1998, Asbjornsen et al., 2005). Woody encroachment of grasslands and savannas can significantly affect the ecohydrological balance at a site by changing the access of plants to water and their total water uptake (e.g., Huxman et al., 2005). Information on how these contrasting annual and perennial plant communities differ in their capacities to access and take up water from different depths in the soil profile can enhance understanding of the ecohydrological implications of large-scale conversion of perennial to annual vegetative cover in Midwestern landscapes, as well as the potential for ameliorating or reversing these changes through the conservation or restoration of native plant communities.

Previous studies have used destructive excavation techniques to characterize rooting depth and distribution for a range of ecosystems. Dahlman and Kucera (1965) determined that 81–87% of the roots and rhizomes in a native prairie in Missouri were located in the upper 25 cm of the soil profile, 6.5–9.0% of the roots were in the upper 25–46 cm, and another 6.0–8.5% were in the lower 46–76 cm. Weaver and Kramer (1932) estimated that bur oaks in eastern Nebraska had a rooting depth down to 4.6 m. An extensive literature review of global root distributions indicated that crops had the shallowest roots (<25 cm), whereas both temperate grasslands and deciduous forests had roots below 50 cm (Jackson et al., 1996). Although data on rooting depth and distributional patterns provide useful insights about water cycling functions of the vegetation, structure and function are not necessarily directly correlated. For example, research has shown that some plants are capable of switching their dominant source of water uptake between deep and shallow levels in the soil profile depending on water availability (Meinzer et al., 1999, Chimner and Cooper, 2004). The amount and spatial distribution of fine and coarse root biomass through the soil profile is therefore not necessarily an accurate reflection of the proportional contribution of soil water from different soil layers obtained by those roots (Thorburn and Ehleringer, 1995, Dawson and Pate, 1996, Midwood et al., 1998). Further, applying excavation techniques to assessing rooting depth at multiple sites quickly becomes time and cost prohibitive (Caldwell and Virginia, 1989).

An alternative approach to assessing source of water taken up by plants is through stable isotope analysis, in which the stable hydrogen (D and H) or oxygen (¹⁸O and ¹⁶O) isotopic composition of stem water is compared with that of potential water sources (e.g., soil water at multiple depths, ground water, and stream water) in order to identify the best ‘match’ based on direct inference (e.g., Jackson et al., 1996, Brundel et al., 1995, Mora and Jahren, 2003). The main assumption of this approach is that roots preferentially take soil water from one depth zone at any given point in time. The possibility of two primary sources for plant water uptake can also be addressed by using a dual-source mass-balance approach, whereby the same isotopic value for stem water is derived by mixing soil waters with higher and lower isotopic values relative to those of stem water (e.g., White et al., 1985, Brundel et al., 1995). These soil waters typically occur at shallower and deeper soil horizons than the one that matches the isotopic value of stem water. Even greater complexity in the determination of water uptake patterns by plants is introduced when we consider that isotopic composition of soil water is variable throughout the soil profile, tending to decrease with depth, but often displaying irregular patterns (e.g., Barnes and Allison, 1988, Jackson et al., 1996, Mathieu and Bariac, 1996, Mora and Jahren, 2003). In this case, multiple possible sources are mixed to produce a single isotope value. Since a unique solution does not exist in this case, the direct inference or dual-source mass-balance approaches often do not produce satisfactory results.

To address the absence of a unique isotope mass-balance solution, Phillips and Gregg (2003) proposed a modified approach involving the calculation of multiple source mass-balance models, which evaluate all feasible contributions of different potential sources. Through this approach, accomplished with the software IsoSource (Phillips and Gregg, 2003), the fractional contribution of each potential source is increased incrementally, and an isotopic mass-balance is performed at each increment to determine if the result compares well with the isotopic composition of stem water. Each time a feasible combination produces the isotopic value of stem water within a given uncertainty or “mass-balance tolerance” (Phillips and Gregg, 2003), those combinations are stored. At the end of the exercise, one obtains the frequency of the number of times a fractional contribution for a given source produces a feasible combination. Thus, results are reported as the distribution of feasible solutions rather than a discrete solution, as occurs when focusing on single values, such as the mean (Phillips and Gregg, 2003). The total number of suitable solutions is dependant on the isotopic values of the sources, the number of sources, the isotopic value of the stem water “mixture”, the increment employed, and the uncertainty level. Ideally, this approach would reduce the feasible fractional contributions of each source to a small range, which would then restrict the number of possible interpretations. In some cases, however, the range of feasible fractional contributions obtained from this approach is large, thus requiring a more qualitative assessment of relative contributions. Although this approach does not provide definitive, unique solutions, it may offer a more realistic and flexible approach, especially given the tremendous complexity and dynamism of ecological processes involved in plant water uptake.

In this study, we applied oxygen isotope techniques together with the multiple-source mass-balance analysis approach (Phillips and Gregg, 2003) to assess depth of water uptake in the soil horizon among dominant plant species growing in contrasting annual (corn) and perennial (prairie, savanna, and woodland) ecosystems in central Iowa. The objectives of this study were threefold: (1) to compare the standard stable isotope analysis approach based on single-source direct inference with the alternative multiple source mass-balance approach and evaluate the utility and limitations of each approach, (2) to determine the probability of the proportional contribution of water from different soil depths to total plant water uptake by dominant species in these contrasting annual and perennial ecosystems, and (3) to enhance understanding of how changes in land use and vegetative cover in the Midwest has altered plant water uptake dynamics and the implications for hydrologic functioning in these landscapes.