The CZ as an Ecological Problem: How the Interplay of Biotic and Abiotic Actors Determines the Functioning of Earth’s Living Skin

Long before the term ‘critical zone’ (CZ) was coined to encompass Earth’s biological and geological features from the top of the vegetative canopy to the depths of circulating groundwater, many scientists have recognized that both biotic and abiotic actors are centrally important for understanding many of Earth’s most fundamental processes. Contemporary CZ scientists continue this legacy. We describe findings that emphasize how life, emphasizing vegetation and microbes, responds to and shapes the physical environment in which it persists, yielding feedbacks for Earth’s climate, primarily through modifications to hydrologic functioning. We focus on the interactions of biota and the physical and chemical features of soil pedons and landscapes as they drive ecosystem-scale hydrologic fluxes. We focus on hydrologically-relevant features because of the long history of individual disciplines telling us about the large-scale importance of these processes, and because of emerging research highlighting the importance of the intersection of these disciplines for projecting future ecosystem functioning on a rapidly changing Earth. The knowledge we spotlight reveals Earth’s CZ as a fundamentally ecological problem.