Nitrate Retention in Headwater Streams: Influence of Riparian Vegetation, Metabolism and Subsurface Processes
Principal investigators:
H. Maurice Valett, Virginia Tech
Jackson R. Webster, Virginia Tech
Patrick J. Mulholland, Oak Ridge National Laboratory
Clifford N. Dahm, University of New Mexico
Christopher G. Peterson, Loyola University, Chicago
Graduate students:
Jack Brookshire, Virgina Tech
Steven Earl, Virginia Tech
student research
Nitrogen (N) loading to the biosphere has increased dramatically since pre-industrial time with strong implications for terrestrial, freshwater and marine ecosystems. In this project, we address the working hypothesis that headwater streams are critical points of N retention across the terrestrial landscape. The project investigates the mechanisms responsible for retention of nitrate-nitrogen (NO3-N) in headwater streams of different regions. Specifically, we address how differences in flow regime, riparian vegetation and ground water-surface water exchange influence retention and how mechanisms of retention vary temporally with season and discharge. To do this, we use 15NO3-N injections as a primary investigative approach.
Because we expect retention processes to differ among catchments that vary in vegetation and climate, we are conducting our experiments at sites ranging from mesic forest catchments of the Southeast with (TN) and without (NC) distinct seasonal variations in stream light level to semi-arid forest catchments in the desert Southwest (NM). Comparisons among sites will address how differences in light availability and allochthonous carbon inputs drive stream metabolism and influence NO3-N retention. Each site includes a pair of streams that differ substantially in surface water-ground water exchange. Comparisons within sites, therefore, will address the role of subsurface metabolic processes and their influence on NO3-N retention.
The first of our research elements focuses on how differences in light availability and allochthonous carbon inputs influence the relative role of autotrophic and heterotrophic retention. Day-time injections of 15NO3-N will be compared with those at night to determine the relative importance of autotrophic and heterotrophic assimilation. In the second element, the relative importance of surface and subsurface processes will be determined using a type of time series analysis of 15NO3-N profiles that we have recently developed and applied to nitrogen solute injection experiments. Finally, because denitrification represents a critical transformation of NO3-N that returns nitrogen to the atmospheric pool, the third research element is dedicated to comparative studies of the mechanisms controlling spatial and temporal variation in rates of denitrification. We will assay for denitrification in hyporheic flow paths using in situ co-injections of a conservative tracer and acetylene. In addition, we will use the recently developed N isotope pairing technique to determine reach-scale measures of denitrification from the longitudinal 15N2 profiles generated during injections of 15NO3-N. The denitrification 15NO3-N injections will be conducted with and without simultaneous additions of labile organic carbon to investigate whether denitrification is carbon limited in streams.
The coupling of hydrologic and biogeochemical processes is an area ripe for the application of modeling tools. A quantitatively-oriented graduate student could immediately apply his/her skills to the improvement and expansion of existing hydrologic models currently in use. Perhaps more exciting, however, is the application of this type of expertise to models relating denitrification, benthic community composition and hydrologic flux. Our understanding of ecosystem N retention would be greatly enhanced by the participation of a researcher with the sorts of skills targeted by the collaboratory.
For more information about this project go to:
http://www.biol.vt.edu/department/research/streamteam/npars/
Student Research:
Nitrate Retention in Headwater Streams: Influence of Riparian Vegetation, Metabolism and Subsurface Processes
Jack Brookshire and Steven Earl
Jack Brookshire
Models of catchment nitrogen (N) retention generally ignore stream processes, assuming that streams are passive conduits for material export. Similarly, models of stream N cycling are rarely placed within a catchment context and largely ignore terrestrial cycling. Understanding long-term effects of anthropogenic N loading necessitates unification of disparate approaches. Terrestrial and aquatic models have traditionally focused on dissolved inorganic N (DIN), yet dissolved organic N (DON) may be a dominant form and play a key role in transformation and long-term retention of N. The objective of this project is to better understand stream contribution to catchment retention of atmospheric N inputs. Modeling is an important component of this research because N cycles along multiple complex pathways and over time scales that are difficult to examine empirically. Phase I of this effort will be to model DIN and DON retention and transformation in headwater streams using nested physical transport and biotic and abiotic transformation models. Models will include simulated injections of DIN, and DON of varying quality, and effects of transient storage, and biotic and abiotic sorption and turnover. Model output will be compared to empirical results from experimental N releases to streams. Phase II of this effort will be to develop a catchment-scale model of N spiraling that couples hillslope and stream transport, short-term uptake processes, and long-term storage and transformation in soils and streams. Output will be compared to empirical results from experimental N additions to various catchment ecosystem compartments.
Steven Earl
Anthropogenic activities involving nitrogen (N) are contributing to eutrophication in marine and fresh waters. Because headwater streams are the primary interface between terrestrial and aquatic systems, they are an important link in the transport of N from enriched landscapes. However, N may be retained or removed through ecological processes occurring within streams. We are investigating the effects of enrichment on the ability of streams to retain N by comparing retention in study streams that span a gradient of N use and concentration. We intend to construct a compartment model to simulate the movement and transformation(s) of N through headwater streams. In addition, we are employing models to estimate hydrologic parameters within our study streams and to estimate biotic uptake parameters during whole-stream N enrichment experiments.
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