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Scott
C. Neubauer
Principal Investigator
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Research
Interests
Biogeochemistry is the study of
mineral cycling and the biotic and abiotic factors that control or
influence that cycling. Sometimes, the terms
“biogeochemistry” and “ecosystem
ecology” are used interchangeably since ecosystem ecology
deals with the flows of energy and materials through natural systems,
and “biogeochemical” factors control these flows.
Research in the Wetland Biogeochemistry Lab at the Baruch Marine Field Lab focuses on the biogeochemical cycling of carbon, nitrogen, and iron in tidal wetlands. Wetland soils and sediments are typically anoxic (no oxygen) and therefore are home to many anaerobic microbes. The delivery of oxygen to wetland soils and sediments is possible at the soil-air and soil-water interfaces. Furthermore, oxygen loss from plant roots can provide a source of oxygen to subsurface soils. Because of the close proximity of aerobic and anaerobic zones, wetlands are biogeochemical hot-spots where many elements cycle between oxidized and reduced forms. For a description of some former and current research projects, keep reading ... |
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 Anaerobic metabolism in tidal wetlands  Spatial and temporal variability
in the importance of microbial metabolic pathways influences
ecosystem-level processes including soil carbon storage, the
regeneration of inorganic nutrients, and the production of
atmospherically-important trace gases. Rates and pathways of microbial
respiration are controlled by supplies of both electron acceptors and
electron donors, and by competition between microbial groups for these
resources.
Our specific research on anaerobic
metabolism in wetlands has focused on how plants can influence pathways
of microbial
metabolism by regenerating electron acceptors via radial oxygen loss
and supplying electron donors in the form of organic carbon. In a tidal
freshwater marsh, Fe(III) reduction accounted for over 90% of total
anaerobic metabolism in June. Over the course of the summer, the
relative importance of Fe(III) reduction decreased, while
methanogenesis increased in significance. We suggest that radial oxygen
loss from plants is critical in regenerating Fe(III) oxides and
allowing Fe(III) reduction to continue. As plants senescence occurs,
rates of radial oxygen loss (and therefore Fe(II) oxidation) decrease,
so Fe(III) reduction becomes limited by the supply of Fe(III). In
addition to this work, we have examined how the balance between three
important anaerobic processes - microbial iron reduction, sulfate
reduction, and methane production - varied along a salinity gradient,
and have done culture-based and molecular analyses to try to relate
biogeochemical rates to microbial community structure.
This work was conducted in tidal
marshes along
the Patuxent River estuary, Maryland in collaboration with Drs. Pat
Megonigal (Smithsonian Environmental Research Center and David Emerson
(American Type Culture Collection).
Neubauer, S.C.,
K. Givler, SK. Valentine, J.P. Megonigal. in press. Seasonal patterns
and plant-mediated controls of subsurface wetland biogeochemistry. Ecology.
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Microbiology of iron cycling The rhizosphere is a dynamic
environment because the release of oxygen from roots (radial oxygen
loss) introduces oxygen into otherwise anoxic wetland sediments and
soils and initiates a series of redox reactions that can affect the
speciation of elements such as iron, carbon, and sulfur. One result of
these redox reactions is the formation of iron oxyhydroxide
precipitates (iron plaque) on plant roots. Iron oxidation kinetics
suggest that most plaque formation in circumneutral environments is
chemically driven, but the discovery of lithotrophic Fe(II)-oxidizing
bacteria raises the possibility that microbes play a significant role
in plaque formation.
We have investigated the microbial
role in iron oxide
formation using pure cultures of strain BrT, a lithotrophic
Fe(II)-oxidizing bacterium isolated from the rhizosphere of Typha
latifolia (cat-tail). In bioreactors, we measured rates of
Fe(II) oxidation in the presence and absence of iron oxidizers and
found that the bacteria could account for up to 50% of total
(biological + chemical) Fe oxidation. In microcosms where radial oxygen
loss from Juncus effusus (soft rush) was the only
oxygen source, we studied Fe(II) oxidation kinetics in the presence and
absence of Fe(II) oxidizers. Data from these experiments are still
being analyzed.
This work was conducted in the lab
in collaboration with
Drs. David Emerson (American Type Culture Collection), Pat Megonigal
(Smithsonian Environmental Research Center) and Johanna Weiss (United
States Geological Service).
Neubauer, S.C.,
D. Emerson, J.P. Megonigal. 2002. Life at the energetic edge: Kinetics
of circumneutral iron oxidation by lithotrophic iron oxidizing bacteria
isolated from the wetland plant rhizosphere. Applied and
Environmental Microbiology. 68:3988-3995. (pdf, 316 KB)
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Responses of tidal marshes to climate change Due to their position at the
interface between terrestrial and marine environments, tidal salt
marshes will be one of the first ecosystems to feel the effects of a
rising sea level. In a manipulative experiment conducted at a mainland
marsh site on the eastern shore of Virginia, a high salt marsh
community was exposed to two disturbances that are expected to result
from a rise in relative sea level: 1) increased frequency of inundation
and 2) enhanced deposition of wrack (mats of dead marsh plant stems and
litter). We showed that flooding and wrack deposition each lowered net
ecosystem carbon accumulation relative to non-manipulated control
plots. Because the sequestration of primary production is important in
maintaining marsh elevation in the irregularly flooded high marsh,
these disturbances may be a first step in initiating the transition
from organic-rich high marsh to mineral-rich low marsh. This work was
conducted in collaboration with David Miller (currently at University
of Maryland) and Dr. Iris Anderson (Virginia Institute of Marine
Science), using experimental plots established by Dr. Bob Christian
(East Carolina University) and students.
Miller, W.D., S.C.
Neubauer, I.C. Anderson. 2001. Effects of sea-level induced
disturbances on high salt marsh metabolism. Estuaries.
24:357-367. (pdf, 640 KB)
An additional effect of rising sea
level is increased salt water intrusion into tidal freshwater marshes.
We have just received an EPA grant to study biogeochemical changes
associated with salt water intrusion. See the project webpage by clicking here.
This project will take place in the Delaware River estuary and will
involve Drs. Melanie Vile and David Velinsky of the Patrick Center for
Environmental Research. The abstract for the project is reproduced
below:
Project Objectives: Tidal
freshwater marshes are often located in areas experiencing intense
urbanization pressure, yet they provide valuable services to coastal
ecosystems by acting as water quality filters (removing nutrients and
sediments), sequestering carbon [C] and phosphorus [P], serving as
nursery habitat for fishes, and buffering storm and flood waters. A
climate change stressor that is unique to tidal freshwater systems is
the intrusion of salt water into environments that have historically
been dominated by freshwater flows. We are especially interested in how
the increase in sulfate concentration associated with salt water
intrusion will affect the biogeochemical interactions that govern the
cycling of C and P in tidal freshwater marshes and affect fluxes of
elements between marshes, tidal waters, and the atmosphere.
Research Approach: We will
implement a novel,
three-phase approach to determine changes in tidal marsh metabolism
[e.g., carbon dioxide and methane gas fluxes and sulfate reduction], C
and P sequestration [sediment deposition and burial], sediment P
speciation, and porewater chemistry at sites along a low-salinity
transitional gradient in the Delaware Estuary. Phase 1 consists of
field observations [as a space-for-time substitute] to assess current
ecosystem services provided by tidal freshwater and low salinity
marshes, and allow us to predict how these services may change as a
result of salt water intrusion. Phases 2 and 3 provide a more detailed
look at specific biogeochemical processes that impact cycling of C, P,
and S. In Phase 2, we will conduct laboratory experiments using marsh
cores exposed to low salinity levels [< 5 psu] to study the
short-term [weeks to months] impact of increased salinity on marsh
sediment C and P biogeochemistry. Phase 3 involves large-scale
manipulations in the field [reciprocal transplanting of cores between
tidal freshwater, oligohaline, and mesohaline marshes] to examine
longer-term ~1-2 yr, ecosystem-level responses of marshes to elevated
salinity.
Expected Results: This research
will improve the
assessment of how ecosystem services provided by tidal freshwater
marshes are likely to respond to predicted changes in climate-induced
sea level rise and salinity. We expect that a small increase in
salinity in tidal freshwater wetland sediments will increase rates of
decomposition [but decrease rates of C burial and emissions of the
greenhouse gas methane], and cause a release of sediment-bound P from
the soils. The results from this project can be used to improve
existing climate change forecast models and will allow appropriate
management to moderate the impacts of future climate change in low
salinity tidal marshes.
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Tidal freshwater marsh C and N cycling Extensive tidal freshwater marshes
are a dominant feature of the upper reach of many estuaries along the
mid-Atlantic coast of North America. To understand the role that these
marshes play with respect to estuarine carbon cycling, we asked two
broad questions: 1) What are the sources of carbon to these marshes?
and 2) What are the ultimate fates of this carbon? We addressed these
questions for a tidal freshwater marsh on the Pamunkey River, Virginia.
Harvest-based estimates of marsh
primary production are
often unreliable due to a lack of information on rates of biomass
turnover and translocation. As an alternative to harvest methods,
fluxes of carbon dioxide have been used to measure photosynthetic rates
for both macrophytes and sediment microalgae. Additionally, carbon
dioxide and methane fluxes measured in the dark can be used to
calculate total respiration rates. Gas flux measurements were used with
a process-based model to calculate annual rates of marsh macrophyte and
microalgal photosynthesis and respiration. The gas flux results were
then combined with biomass harvest and literature data to create a
conceptual mass balance model of macrophyte-influenced carbon cycling
in the marsh. Annually, total marsh respiration exceeded gross
photosynthesis, suggesting the input of additional carbon source(s) to
the marsh.
Because there is a large decrease
in water column
turbulent energy as marshes are flooded, tidal marshes are often sites
with high sediment deposition rates. Short-term sediment deposition
rates (biweekly to monthly) measured using sediment collection tiles
were spatially and temporally variable but were sufficient to balance
the combined effects of marsh respiration and relative sea level rise.
Sediment core inventories of the radioisotope beryllium-7 showed that
the spatial patterns of sediment deposition were not due to erosion and
redistribution within the marsh. Accretion rates calculated from
cesium-137 and carbon-14 dating were substantially less than annual
deposition rates, with a decrease in accretion rate with increasing
time scale. The metabolism of a labile sediment fraction can explain a
portion of this observed decrease in accretion rate, with the remainder
likely due to periodic storm-induced erosion and historical variability
in sediment deposition rates.
Seasonally, the concentration of
dissolved inorganic
carbon (DIC) was measured in a marsh tidal creek to quantify the
exchanges between the marsh and estuary. At low tide, DIC
concentrations were 1.5 to 5-fold enriched relative to high tide
concentrations, indicating an input of DIC from the marsh. On an
estuary wide scale, the export of marsh-derived DIC could explain a
significant portion of excess DIC production in the adjacent York and
Pamunkey River estuaries and indicates that estuarine DIC
concentrations in excess of conservative mixing between freshwater and
marine end members does not necessarily indicate that an estuary is net
heterotrophic.
To date, most studies that have
examined nutrient
exchanges in tidal freshwater marshes have done so in highly polluted
or eutrophic systems. In parallel with studies on carbon cycling
(described above), we examined nitrogen cycling in a relatively
pristine (i.e. low nutrient) tidal marsh on the freshwater Pamunkey
River, Virginia. A process-based mass balance model of N cycling for
this system revealed that nitrogen cycling in the system was largely
conservative. Gross mineralization of organic N was the largest
nitrogen flow and accounted for more than enough N to support total
macrophyte and microalgal primary production. Efficient microbial
utilization of porewater ammonium helped retain N within the marsh.
Uptake of N by tidal marshes may be important in controlling estuarine
nutrient concentrations, but may play only a small role in supporting
marsh primary production.
S.C. Neubauer,
I.C. Anderson, B.B. Neikirk. in press. Nitrogen cycling and ecosystem
exchanges in a Virginia tidal freshwater marsh. Estuaries.
S.C. Neubauer, I.C. Anderson. 2003. Transport of dissolved inorganic carbon from a tidal freshwater marsh to the York River estuary. Limnology and Oceanography. 48:299-307. (pdf, 1.1 MB) S.C. Neubauer, I.C. Anderson, J.A. Constantine, S.A. Kuehl. 2002. Sediment deposition and accretion in a mid-Atlantic (U.S.A.) tidal freshwater marsh. Estuarine, Coastal, and Shelf Science. 54:713-727. (pdf, 320 KB) S.C. Neubauer, W.D. Miller, I.C. Anderson. 2000. Carbon cycling in a tidal freshwater marsh ecosystem: a carbon gas flux study. Marine Ecology Progress Series. 199:13-31. (pdf, 1.8 MB) |
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Last modified by Scott Neubauer, 27 October 2005 | |||||||||||||||||||||||||
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