USING THE STABLE ISOTOPE COMPOSITION OF PRECIPITATION, SOIL POREWATER, GROUNDWATER, AND PLANT XYLEM WATER TO DELINEATE SOURCES OF PLANT WATER UPTAKE IN THE NEBRASKA SANDHILLS

The spatial and seasonal dynamics of water uptake by trees and grasses in semi-arid savannas affect interspecific plant competition, ecosystem evapotranspiration, and groundwater recharge. The stable oxygen and hydrogen isotope compositions of precipitation, soil porewater (to 3 meters depth), groundwater and plant xylem water were used to quantify the spatial utilization of soil water by trees (Pinus ponderosa and Juniperus virginiana) and dominant C₄ grasses (Schizachyrium scoparium and Panicum virgatum) in the Nebraska Sandhills. Water samples were collected monthly for one year and analyzed to determine their δ¹⁸O and δ²H signatures. Water was extracted by azeotropic distillation prior to mass spectrometer analysis. The δ¹⁸O composition of precipitation was consistent with regional monthly averages, varying seasonally as a function of temperature. Oxygen isotope profiles of soil porewater reflected seasonal variations in the isotope composition of local precipitation within the upper profile (0-150 cm), with values ranging from -4.9 to -19.8 ‰, and approached the groundwater value (-11 ‰) at depth (150-300 cm). Grass xylem water δ¹⁸O values ranged from 0.47 to -7.3 ‰, tracking the values of recent precipitation and soil water recharge in the upper 50 cm. In contrast, tree xylem water δ¹⁸O values ranged from -6.9 to -14.5 ‰, suggesting two patterns when compared to soil moisture values (measured continuously to 1 m depth): (1) under conditions of adequate soil moisture, trees use both shallow soil water derived from precipitation, and deeper groundwater, and (2) during periods of water stress, the δ¹⁸O composition of xylem water in the trees is close to that of groundwater, suggesting that water is drawn only from depth. Thus, competition among grasses and trees is tied closely to soil moisture availability in the upper vadose zone. When soil moisture is available in the upper vadose zone, trees and grasses compete, but when soil moisture is depleted near the surface, as occurs frequently in the Sandhills, trees use water from depth and avoid grass competition. Presently, work is underway to determine what impact this behavior might have on evapotranspiration and recharge calculations across the Sandhills region.