A Gray Box Approach to Nutrient Budgets: Watershed Subsystems and the Biogeochemical Continuum.

Small watershed approaches to nutrient budgets have revealed striking patterns illustrating biotic control of material retention. Budgets addressing nitrogen (N) pools and fluxes have become increasingly relevant as anthropogenic loading of N has climbed during the course of the 20^th century. Conceptually, the small watershed approach views streams and associated terrestrial environments as part of a single functional entity. Historically, however, watershed losses have been measured as dissolved and particulate output at the weir, and streams have been viewed as inert pipes. Recent recognition of in-stream processing, multiple watershed components, and the importance of temporal integration has promoted an opening of the traditional black box. Here we propose a gray box approach that recognizes the existence of critical subsystems within watersheds and emphasizes the importance of interfaces to advance studies addressing internal controls of nutrient budgets. We introduce the concept of a biogeochemical continuum that argues for integration of terrestrial and aquatic subsystems to understand control points responsible for characteristic nutrient budgets.