Overarching Scientific Questions Pertaining to the Critical Zone: Challenges to the Soil and Geologic Sciences Community.

The Critical Zone (CZ) is a holistic framework for integrated studies of water with soil, rock, air, and biotic resources in the near-surface terrestrial environment.  This most heterogeneous and complex region of the Earth ranges from the vegetation top to the aquifer bottom, with a highly variable thickness globally and a yet-to-be clearly defined lower boundary of active water cycle.  Interfaces among different compartments in the CZ are critical, which provide fertile ground for interdisciplinary research.  The reconciliation of coupled geological and biological cycles (vastly different in space and time scales) is essential to understanding the complexity and evolution of the CZ. Irreversible evolution, coupled cycling, interactive layers, and hierarchical heterogeneity are among the overarching scientific issues pertaining to the CZ, suggesting that forcing, coupling, interfacing, and scaling are grand challenges for advancing CZ science. The pedosphere is the foundation of the CZ, which represents a geomembrance across which water and solutes, as well as energy, gases, solids, and organisms are actively exchanged with the atmosphere, biosphere, hydrosphere, and lithosphere, thereby creating a life-sustaining environment.  Our capability to predict the behaviour and evolution of the CZ in response to changing environment can be significantly improved if cross-site scientific comparisons, evolutionary treatment of organized complex systems, and deeper insights into the CZ can be made.  In this presentation, we will highlight the opportunities and challenges facing both soil and geologic sciences community. We will also show case Critical Zone Observatories in the U.S. and alike around the world that are operated at the watershed scale and are expected to showcase integrated, interdisciplinary, and multiscale efforts that will advance our ability to forecast Critical Zone structures and functions across scales.