Earth's Critical Zone and hydropedology: concepts, characteristics, and advances

The Critical Zone (CZ) is a holistic framework for integrated studies of water with soil, rock, air, and biotic resources in terrestrial environments. This is consistent with the recognition of water as a unifying theme for research on complex environmental systems. The CZ ranges from the top of the vegetation down to the bottom of the aquifer, with a highly variable thickness (from <0.001 to >10 km). 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 to create a life-sustaining environment. Hydropedology – the science of the behaviour and distribution of soil-water interactions in contact with mineral and biological materials in the CZ – is an important contributor to CZ research. This article reviews and discusses the basic ideas and fundamental features of the CZ and hydropedology, and suggests ways for their advances. An "outward" growth model, instead of an "inward" contraction, is suggested for propelling soil science forward. The CZ is the right platform for synergistic collaborations across disciplines. The reconciliation of the geological (or "big") cycle and the biological (or "small") cycle that are orders of magnitude different in space and time is a key to understanding and predicting complex CZ processes. Because of the layered nature of the CZ and the general trend of increasing density with depth, response and feedback to climate change take longer from the above-ground zone down to the soil zone and further to the groundwater zone. Interfaces between layers and cycles are critical controls of the landscape-soil-water-ecosystem dynamics, which present fertile grounds for interdisciplinary research. Ubiquitous heterogeneity in the CZ can be addressed by environmental gradients and landscape patterns, where hierarchical structures control the landscape complex of flow networks embedded in mosaics of matrices. Fundamental issues of hydropedology are linked to the general characteristics of the CZ, including (1) soil structure and horizonation as the foundation of flow and transport characteristics in field soils; (2) soil catena and distribution pattern as a first control of water movement over the landscape; (3) soil morphology and pedogenesis as signatures of soil hydrology and soil change; and (4) soil functional classification and mapping as carriers of soil hydrologic properties and soil-landscape heterogeneity. Monitoring changes in the crucible of