A warmer, more CO2-rich climate amplifies hydrological disconnections within soil water

Shallow subsurface soil water storage is conventionally depicted as a well-mixed reservoir whereby newly fallen precipitation displaces or mixes with existing storage en route to streams—linking transpiration and rootzone storage to streams. Contrary to this core mixing assumption, mounting evidence suggests separations can arise in vadose (unsaturated) zone pore space or flow paths, yet the mechanisms remain poorly constrained. Here, we discuss recent studies from a climate manipulation experiment which use isotopic tracer and numerical techniques to understand the ecohydrological impact of future climactic conditions (elevated atmospheric CO₂ and air temperature) on soil water transit and cycling in a temperate grassland. Whereas soil water in this ecosystem typically remained well-mixed, sustained exposure to future climate triggered separations across pore space and between soil horizons—exacerbated by experimental drought. Further, under this future drought scenario the grassland conserved water by restricting evapotranspiration at lower atmospheric water demand than drought exposure in ambient climate. Our results suggest that future climatic conditions may amplify subsurface disconnections in soil water, constraining grassland water use and altering the ecohydrological trajectory of these ecosystems.