Subsurface interfaces, such as those found between soils and gravel beds, across water table surfaces, at the regolith-bedrock transition, between ground- and surface water, and along fine-grained sediment lenses, are abundant in Earth’s Critical Zone. Contrasting permeability, redox status, and nutrient and contaminant loads are juxtaposed across these interfaces, creating conditions ideal for exchange of oxygen, carbon, and reactive solutes, often driven by fluctuating hydrologic conditions. These zones of reactivity profoundly influence the biogeochemical function of floodplains, hillslopes, and river corridors, mediating carbon retention or release and the compositions of ground- and surface water. Yet, in many cases we know very little about the biogeochemistry and hydrology of these interfaces. We invite observational, experimental, and modeling contributions focused on coupled hydro-biogeochemical processes across subsurface interfaces. We encourage contributions on transport, microbial communities and metabolism, geochemical reaction networks, and fluxes of oxygen, carbon, nutrients, and contaminants, and their impacts on watersheds.