IRCEB: Nitrate Uptake and Retention in Streams: Mechanisms and Effects of Human Disturbances from Stream Reaches to Landscapes

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

Human activities are flooding the biosphere with nitrogen (N), changing the biogeochemistry of terrestrial and aquatic environments. The fate of much of the anthropogenic N load to the biosphere is uncertain because mass balance studies of major watersheds show that most of the added N is not exported to the oceans but is missing, having disappeared from our accounting somewhere within these watersheds. Most of the inorganic N entering streams is in the form of nitrate. Traditionally, nitrate has been thought to flow freely downstream to coastal ecosystems. Recently, however, mass balance analyses for the Mississippi drainage have shown that large quantities of nitrate are lost as water travels through its tributary streams. This conclusion is consistent with earlier findings that smaller streams are most retentive of ammonium. This project will be an intensive, inter-site study of the fate of nitrate in streams. The investigators will evaluate whether streams are indeed important sites of N retention in the landscape and how human disturbances affect that retention. Field 15N-tracer experiments will be used to determine the rates and factors controlling nitrate uptake and retention in relatively pristine streams and in disturbed (agricultural and urban) streams. The experimental results will be used to develop a process-based model of nitrate retention in streams. The results will be extended to larger spatial scales using GIS approaches to predict nitrate retention in large river basins at each of eight study sites. These predictions will be tested with synoptic field measurements of stream nitrate concentrations within each basin. The validated river basin model will be an important land-use planning tool for controlling the N loading of lakes and coastal ecosystems. The work will have broader implications to society by explicitly evaluating the effects of human disturbance on N retention in stream networks, contributing the new knowledge needed for a more sustainable management of watersheds.