Trees, the most successful biological power plants on earth, build and plumb the critical zone (CZ) in ways that we do not yet understand. To encourage exploration of the character and implications of interactions between trees and soil in the CZ, we propose nine hypotheses that can be tested at diverse settings. The hypotheses are roughly divided into those about the architecture (building) and those about the water (plumbing) in the CZ, but the two functions are intertwined. Depending upon one's disciplinary background, many of the nine hypotheses listed below may appear obviously true or obviously false. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil where that underlying substrate is fractured or pre-weathered. (2) In settings where the thickness of weathered material, H, is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, h, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where h ≪ H and the rates of uplift and erosion are low, the uptake of phosphorus into trees is buffered by the fine-grained fraction of the soil, and the ultimate source of this phosphorus is dust. (5) In settings of limited water availability, trees maintain the highest length density of functional roots at depths where water can be extracted over most of the growing season with the least amount of energy expenditure. (6) Trees grow the majority of their roots in the zone where the most growth-limiting resource is abundant, but they also grow roots at other depths to forage for other resources and to hydraulically redistribute those resources to depths where they can be taken up more efficiently. (7) Trees rely on matrix water in the unsaturated zone that at times may have an isotopic composition distinct from the gravity-drained water that transits from the hillslope to groundwater and streamflow. (8) Mycorrhizal fungi can use matrix water directly, but trees can only use this water by accessing it indirectly through the fungi. (9) Even trees growing well above the valley floor of a catchment can directly affect stream chemistry where changes in permeability near the rooting zone promote intermittent zones of water saturation and downslope flow of water to the stream. By testing these nine hypotheses, we will generate important new cross-disciplinary insights that advance CZ science.

The role of groundwater as a resource in sustaining terrestrial vegetation is widely recognized. But the global prevalence and magnitude of groundwater use by vegetation is unknown. Here we perform a meta-analysis of plant xylem water stable isotope (δ²H and δ¹⁸O,n = 7367) information from 138 published papers – representing 251 genera, and 414 species of angiosperms (n = 376) and gymnosperms (n = 38). We show that the prevalence of groundwater use by vegetation (defined as the number of samples out of a universe of plant samples reported to have groundwater contribution to xylem water) is 37% (95% confidence interval, 28–46%). This is across 162 sites and 12 terrestrial biomes (89% of heterogeneity explained;Q-value = 1235;P < 0.0001). However, the magnitude of groundwater source contribution to the xylem water mixture (defined as the proportion of groundwater contribution in xylem water) is limited to 23% (95% CI, 20–26%; 95% prediction interval, 3–77%). Spatial analysis shows that the magnitude of groundwater source contribution increases with aridity. Our results suggest that while groundwater influence is globally prevalent, its proportional contribution to the total terrestrial transpiration is limited.

Background: Sustainable Development Goal (SDG) 6 envisions a future where everyone has access to clean water and sanitation. Yet, as 2030 looms closer, the complexity of achieving this target becomes apparent, with issues far surpassing basic water infrastructure and utility challenges. The underlying problems lie in broader spheres such as governance, policymaking, and financing. Main body: The global landscape of water management is marked by complexities that transcend the operational troubles of water utilities. Financial sustainability is a monumental task. And while it is true that water utilities struggle with revenue generation, the broader picture reveals systemic challenges. The true cost of water provision often extends to ecosystem services such as watershed protection. Often, these services are not internalized in the revenue models of utilities but are typically subsidized by governments or simply not considered. Balancing affordability for users with cost recovery for service providers, however, is not just an arithmetic exercise. It is also a question of equitable policies. Non-revenue water (NRW), resulting from physical losses such as leaks, theft, and inaccurate [or lack of] metering, exacerbates existing financial strain. Annual NRW losses are estimated at an astonishing 126 billion cubic meters, costing roughly USD 39 billion. But at the most fundamental level of achieving SDG 6 is misgovernance. Effective water governance demands consistent policies, coherent collaboration among diverse stakeholders, and comprehensive strategies that cater to specific regional contexts. Current models often suffer from fragmented policies, inadequate public-private partnerships, and weak engagement mechanisms. A glaring gap exists between academic advancements in water management and their practical implementation in policymaking. Moreover, international cooperation, while vital, reveals an unequal landscape in knowledge exchange. Knowledge transfer is often skewed, favoring dominant nations while sidelining voices from the Global South. This emphasizes the need for an inclusive, equitable, and context-specific global cooperation model. Conclusion: The road to realizing SDG 6 is multifaceted, and while on-the-ground solutions are essential, the real success lies in addressing the foundational challenges. This requires innovative financial solutions, reimagining water governance structures, and ensuring all voices, especially from the Global South, are heard and integrated into global policies. As 2030 nears, it is the synergy of governance, finance, and technology that will ultimately make clean water and sanitation a reality for all.