Drought in a warmer, CO-rich climate restricts grassland water use and soil water mixing.

Soil water sustains terrestrial life, yet its fate is uncertain under a changing climate. We conducted a deuterium labeling experiment to determine whether elevated atmospheric carbon dioxide (CO), warming, and drought impact soil water storage and transport in a temperate grassland. Elevated CO created a wetter rootzone compared with ambient conditions, whereas warming decreased soil moisture. Soil water remained well mixed in all global change treatments except for summer drought combined with warming and elevated CO. These combined treatments caused the grassland to conserve water and restricted soil water flow to large, rapidly draining pores without mixing with small, slowly draining pores. Our results suggest that drought in a warmer, more CO-rich climate can severely alter grassland ecohydrology by constraining postdrought soil water flow and grassland water use.