Grazing impacts on hydraulic lift in a tallgrass prairie

Background/Question/Methods

Hydraulic lift (HL), the passive night-time movement of water through plant roots from deep soil layers to drier shallow soil, is an ecologically important process that can influence plant growth and survival, species interactions, and ecosystem biodiversity. Although HL has been observed in savanna and forest ecosystems, it is currently unknown if HL occurs in grazed tallgrass prairies. To address this knowledge gap, we evaluated (1) whether HL occurs in shrubs, forbs, and grasses at the Konza Prairie Biological Station and (2) if grazing by Bison bison influences HL in these species. Night-time water movement through soil associated with HL was traced using stable isotope ratios of soil and plant water. We collected non-photosynthetic plant tissue and adjacent soil cores at dusk (1800 hours) and the following dawn (600 hours) from a C₃ shrub (Rhus glabra), a C₃ sub-shrub (Amorpha canescens), a C₃ forb (Vernonia baldwinii), and a C₄ grass (Andropogoni gerardii) in grazed and non-grazed watersheds during July and August 2013. Water was extracted from plant tissue and soil cores using a cryogenic vacuum distillation method and analyzed for δD and δ¹⁸O signatures. HL was identified by comparing diurnal changes in the signature of xylem and soil water.

Results/Conclusions

We found evidence of HL during both months, although this varied among species and locations. For example, the isotopic signature of the water in the soil profile next to plants differed between diurnal time points. Water collected from shallow sections of the soil core was significantly more depleted in δ¹⁸O during the morning than it was in the evening, suggesting that water deeper in the soil profile moved to more shallow soil layers throughout the course of a single night. These differences in δ¹⁸O occurred in soil adjacent to A. canescens, V. baldwinii, and R. glabra, but not in soil adjacent to A. gerardii, suggesting that this grass species does not hydraulically lift water. Rhus glabra, A. canescens and V. baldwinii xylem water also contained a greater proportion of deep water at dawn compared to dusk, although this occurred primarily in the ungrazed watershed and varied in magnitude between months. Thus, shrubs and forbs do hydraulically lift water in this system, although HL is reduced by grazing and varies across a growing season. This has the potential to enhance water and nutrient availability, daytime transpiration, and physiological functioning during drought for lifting plants in non-grazed tallgrass prairies.