DIFFERENCES IN SOIL-WATER USE BY ANNUAL BROOMWEED AND GRASSES

The use of water in the upper 1 m of the soil profile by 3 common herb aceous species of the southern Great Plains was examined by labeling s oil water with (H2O)-H-2 and (H2O)-O-18, Uptake of labeled water from the 15 cm depth was approximately equal for all species, However, wate r uptake from the 75 cm depth was significantly greater by annual broo mweed [Amphiachyris dracunculoides (DC.) Nutt] than either sideoats gr ams [Bouteloua curtipendula (Michx.) Torr] or curlymesquite [Hilaria b elangeri (Steud.) Nash]. Although both grasses had greater root length density than annual broomweed at the 75 cm depth, annual broomweed's rate of water extraction from the 75 cm depth was nearly twice that of sideoats grams or curlymesquite, Greater access to and more rapid uti lization of deeper soil water by annual broomweed relative to the gras s species may partially explain annual broomweed's success at invading grasslands and reducing grass production in semi-arid rangelands.