Channel complexity, organic matter storage and N processing

 We are interested in understanding the linkages between carbon and nitrogen cycling in heavily modified urban streams. We hypothesize that channel complexity (or the ability of the stream channel to trap and retain materials) controls organic matter storage, and that organic matter storage, in turn, determines the capacity of the stream ecosystem to process inorganic nitrogen. Urban stream ecosystems are ideal study systems for this type of research since many urban stream channels have been channelized and simplified, while at the same time large-scale efforts are being made to restore channel and habitat complexity in urban streams in order to prevent downstream nutrient and sediment export. Thus, urban streams can be found across an artificially extended gradient of channel complexity. We aim to exploit this exaggerated gradient to test our hypotheses about the relationship between channel structure and organic matter storage, and between organic matter storage and N processing.

 Elizabeth Sudduth and Emily Bernhardt

Trajectories of ecosystem recovery in urban riparian wetlands

 Intact riparian zones provide an important ecosystem service by removing nutrients, particularly nitrogen (N) and phosphorus (P) from groundwater and surface runoff, thus improving surface water quality. Because streams and riparian zones in many urban areas are degraded and have diminished ecological functions, stream and floodplain restoration is becoming a significant part of water quality management strategies. Understanding how riparian zones regain ecological functions following restoration is critically important to setting realistic timeframes and performance criteria for restoration.  We are using restoration sites as experiments to test theories of ecosystem development as vegetation and soil processes become re-established over time.  By comparing a series of restored streams implemented more than a decade apart and comparing them to nearby reference (non-urban) streams and highly degraded, unrestored streams, we can test basic biogeochemical theory and contribute immediate information to guide the practice of restoration.  We expect that restoration projects that lead to more variable water table depths and less incised channels will have higher capacities for riparian nutrient retention.  We expect that the time since restoration will explain differences in ecosystem functions in restored riparian zones, as vegetation and soil processes begin to approach those of natural riparian zones.  Abiotic mechanisms of nutrient retention (e.g., P sorption) will likely be more important in younger restored sites, while biotic processes (e.g., microbial assimilation, denitrification, plant uptake) will be increasingly important controls on nutrient biogeochemistry in older restored sites.

 Jen Morse & Emily Bernhardt

Carbon in the rhizosphere
Effects of elevated CO2 and soil fertility on rhizodeposition

 Our work in this area is in conjunction with the Duke Forest FACE experiment in which intact loblolly pine plantation plots have been fumigated with CO2 gas at 200ppm over ambient CO2 concentrations since 1996. The goals of this research effort are to determine: 1) whether loblollies alter the amount or composition of rhizosphere carbon flow (RCF) in response to elevated CO2; and 2) whether RCF increases plant access to limiting nutrients. The purpose of this research is to determine whether CO2 induced changes in root exudation can help trees maintain heightened productivity by allowing them to access new or less labile forms of nutrients.

 Rich Phillips and Emily Bernhardt.

Hyporheic N processing
Effects of hydrology and carbon delivery

 We are exploring new research ideas that focus on the role of buried wood, live root biomass and active root exudation in affecting N processing rates along hyporheic flowpaths in the Nyack River floodplain, Glacier National Park, MT. We have begun research in the field as well as in large hyporheic mesocosms. We will examine the role of buried and exuded carbon on rates of denitrification along hyporheic flowpaths in this unconstrained and unpolluted large river floodplain ecosystem.  Field and mesocosm experiments and monitoring will be used to link existing hydrologic models for the site with biogeochemical models.

 Emily Bernhardt, Ric Hauer, Jack Stanford and Brian Reid (University of Montana & the Flathead Lake Biological Station) and Geoff Poole (EcoMetrics)

Dissolved Organic Nitrogen in stream ecosystems
Controls on quantity and quality

 For the last year we have been examining spatial patterns of DON concentration throughout the Smoky Mountains and temporal dynamics of DON in Walker Branch at ORNL.  We have documented large changes in DON concentrations across gradients of inorganic nitrogen deposition.  Controls on DON concentration appear to shift seasonally and over the course of storm events.  We are actively pursuing funding to pursue questions about the bioavailability, production and consumption of DON in stream ecosystems.

 Emily Bernhardt, Brian Roberts and Pat Mulholland (Oak Ridge National Laboratory) and Jack Brookshire (Virginia Tech).

National River Restoration Science Synthesis (NRRSS)

Learn more about the team and our efforts at our website.

 This large-scale effort to summarize the status of river restoration in the U.S. was initiated in August 2002. Our goal is to bring together information on restoration projects throughout the nation in order to answer several fairly basic questions such as: How much is being spent on restoration annually? How do restoration efforts vary between regions of the U.S.? How often are restoration projects monitored? What proportion of projects funded with "restoration" money do not have ecological or geomorphic restoration as a goal?  It is currently impossible to answer even these simplistic questions about the state of U.S. River Restoration efforts, since information on restoration efforts is maintained by numerous agencies, at all levels of government (local, state, regional, federal), and recordkeeping is often not a priority for these entities. During the first stage of the project the NRRSS team assembled a common language database of ~37,000 projects from throughout the U.S. (see our paper in Science). In stage two, we are following up on a random subsample of the projects, interviewing project managers to learn more about project goals, limitations, successes and failures, and the role of science in informing restoration practice.

 Emily S. Bernhardt, Elizabeth Sudduth and Remi Treuer all work as part of the NRRSS effort here at Duke

Long-term change in stream ecosystems

 There has been very little work to date on ecosystem development in lotic ecosystems, despite a tremendous body of work in temperate terrestrial ecosystems. At the Hubbard Brook Experimental Forest there has been a change in how stream ecosystems process nitrogen. In the 1970's Joanna Richey and Bill McDowell were unable to measure any uptake of NO3 from streamwater, instead stream ecosystems served as sources of inorganic nitrogen through high rates of nitrification. In the late 1990's, my work with Bob Hall and Gene Likens showed that HBEF streams actively process NO3, and can dramatically reduce watershed export of N. To test whether these ecosystems have changed, Bill McDowell and I repeated in 2000, experiments that he conducted in the late 1970's, adding leaf leachate and determining the effect on DIN concentrations. We found that while adding leachate C to streams in the 1970's had no impact on stream N processing, the same releases in the 1990's dramatically stimulated NO3 uptake. With the support of the A.W. Mellon foundation we held a workshop of all stream researchers that had conducted research at HBEF since the early 1960's. The purpose of the workshop was to synthesize data and knowledge about how HBEF streams have changed over the past 40 years.  We demonstrated that changes in HBEF streams over the period of record could have important implications for interpreting the long-term record of streamwater N losses and propose new approaches for the long-term monitoring of watershed studies that will allow us to better understand stream impacts on N cycling in the future (see resulting paper in BioScience).

 Emily Bernhardt with Gene Likens (IES), Robert Hall, Jr. (U WY), Bill McDowell (UNH), Dana Warren (Cornell) 

Thermodynamic and hydrologic controls over material fluxes and biogochemical transformations in a large-scale restored riverine wetland.

We are using a dynamic flooding gradient, created by a large-scale (400 ha) wetland restoration project, to examine the role of hydrology and vegetation on trace gas emissions and transformations of C, N, and P.  We hypothesize that the biogeochemical patterns along individual flow paths are determined by the thermodynamically-constrained metabolism of microorganisms and supplies of electron donors and acceptors, which are in turn influenced by hydrology.  The restoration of former agricultural areas presents the potential for water quality improvements.  Increased P sorption and denitrification in restored wetland soils can have an important role in improving surface and ground water quality.  Flooding of nutrient-rich agricultural soils, however, can potentially lead to an initial pulse of dissolved nutrients.  Furthermore, denitrification and other soil microbial processes represent major biogenic sources of trace gases to the atmosphere.  Emissions of nitrous oxide (N₂O) and methane (CH₄) have been well documented in certain natural and human-dominated wetland environments, such as riparian forests, rice paddies, and constructed treatment wetlands.  Using data from monitoring, field and laboratory experiments we hope to create a linked hydrologic and biogeochemical model that will extrapolate from patch-scale measurements to landscape processes. This model will provide new insights into the mechanisms of the environmental attributes of biogeochemical “hot spots” or “hot moments”.

Marcelo Ardón, Jen Morse, Alison Appling, Emily Bernhardt, Geoff Poole, Ashley Helton and Martin Doyle.