PARENT SESSION
Symposium 4: Ecosystem ecology at the watershed scale: Linking biogeochemical cycles across the terrestrial - aquatic divide
Organized by: ES Bernhardt and MH Valett
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 517 A, Level 5, Palais des congrès de Montréal

Controls on nitrogen export from terrestrial ecosystems.

Goodale, Christine ^*,1, ¹ Department of Ecology & Evolutionary Biology, Ithaca, NY, 14853

ABSTRACT- Terrestrial ecologists have long used stream chemistry as an indicator of terrestrial processes, particularly long timescale processes that reflect ecosystem carbon-nitrogen interactions such as nutrient limitation, ecosystem succession, or response to anthropogenic nitrogen deposition. In most cases, stream nutrient export fluxes can be measured far more precisely than fluxes of internal cycling within the terrestrial system (e.g., mineralization, plant uptake). Patterns of stream N export across hundreds of forest and heathland catchments in Europe and North America indicate that atmospheric N deposition has little effect on dissolved organic nitrogen export, but is a major driver of stream nitrate loss. However, there is considerable variation in nitrate export among streams receiving similar rates of N deposition. Multiple factors have been suggested to explain this variation. These factors fall into broad categories of terrestrial factors that affect organic matter quality, quantity, and accumulation rate (species composition, successional status, and soil C:N ratio), and factors that affect the routing and processing of water as it flows through the catchment. Going inside the terrestrial box, experimental fertilization and laboratory studies have demonstrated that soil processes - both biotic and abiotic - are fundamental controls on upland retention of inorganic nitrogen. Nonetheless, the precise mechanisms and longevity N retention in soil organic matter remain major uncertainties. Furthermore, hydrologic routing may bypass some of these soil retention mechanisms, providing major opportunities for collaboration across disciplines. A grand challenge to all three disciplines remains the difficulty in understanding multi-year to decadal patterns of stream nitrate export, which is the timescale of greatest relevance in determining policies to reduce N emissions.

Key words: nitrogen retention, nitrate export, carbon-nitrogen interactions