John Navaratnam (PHD Student):

My work focuses on the microbial ecology of floodplain forest ecosystems in the southeastern US.  In particular, I am interested in understanding the microbial community structure in floodplain soils and their contribution to phosphorus retention and transformation processes within these wetlands.  Microbial dynamics are an important and yet comparatively unexplored component of phosphorus retention and transformation in floodplain wetlands.  Floodplain forests are known to play key roles in the retention and transformation of orthophosphate (PO₄³⁻) received during flooding, thereby improving water quality in aquatic ecosystems downstream.  Phosphates can be transformed to organic P prior to export, or retained within the floodplain forest through sediment deposition, soil adsorption, plant uptake, and/or microbial immobilization. Below are some of the specific projects that I currently am involved in:

 1. Phosphorus chemistry of inflow vs. outflow floodwaters: This work entails the use 31P-Nuclear Magnetic Resonance (NMR) Spectroscopy to examine the forms and relative proportions of inorganic and organic phosphorus in floodwaters.

 2. Identifying the dominant mechanisms of phosphorus retention and transformation: This involves a laboratory mesocosm approach and the simulated flooding of intact soil cores with radioactive labeled (33 and 32-Phosphate) tracers to understand the fate of phosphorus in floodplain soils.  In addition to measuring the phosphorus chemistry in these soil cores, the microbial community structure is analyzed using molecular techniques.

 3. Linking microbial diversity to biogeochemical function: As part of this research, I have used molecular techniques to assess the microbial (archaeal, eubacterial, and fungal) community composition in ridge and swale microsites in alluvial and blackwater study sites. I have begun characterizing the microbial community structure using a combination of total soil (genomic) DNA extractions, PCR amplification, plasmid cloning, denaturing gradient gel electrophoresis (DGGE), and sequence analysis.  Preliminary data suggests spatial variation in microbial community composition both within and between sites, and species diversity indicating differences in biogeochemical (C, N, S, and P) processing that have potential importance for achieving our overall goal of understanding P retention and transformation mechanisms in floodplain forests.

The objectives of my study are to 1) examine in more detail the relationship between productivity and nutrient availability in the southeastern floodplain forests as a function of floodplain type (alluvial vs. blackwater) and the age of underlying parent materials (old vs. young); 2) test Lockaby and Walbridge's (1998) model of the relationship between productivity and nutrient availability in southeastern forested floodplains. I hypothesize that productivity will be greatest in forests where the availability of N and P is closely balanced, but will decline when either nutrient is in excess.  Alluvial systems will tend to be N limited, blackwater systems will tend to be P limited. Because N availability is expected to be greater in older parent materials, while P availability is expected to be greater in younger parent materials. Alluvial forests on older parent materials should be more productive than those on younger parent materials while the reverse is expected to be true for blackwater floodplain forests.

Sarah Kelly (Undergraduate researcher):

Sarah will be completing a field study this summer 2005, continuing Heather Fredericks work on the remaining four sites. She will assess the community composition of each site, measure aboveground biomass using DBH, and collect foliar samples at the end of June for nutrient analyses. She will be assisting in a nutrient enrichment study using root cores in these four sites also.

Data

Floodplains are important components of river ecosystems and are intimately linked to the efficient functioning of rivers. Among other processes, floodplains contribute to shaping and maintaining water quality. Indeed, significant amounts of nutrients (N, P), sediment, and organic matter originally transported by the river are deposited in floodplains during floods. Nutrients (N, P), sediments and organic matter can also be supplied to the floodplain via upland runoff and subsurface flow. Eventually, the alluvial floodplain acts as an interface between terrestrial and aquatic systems where inorganic and organic nutrients have a chance to be removed from incident surface, subsurface, and ground waters.

J.C. Clement:

Indeed, nitrate (NO₃^-), ammonium (NH₄⁺) and dissolved organic nitrogen forms (DON) may interact with floodplain sediments where many processes occur such as: denitrification, adsorption on mineral or organic matters, uptake and immobilization by microorganisms, algae and plants, and volatilization. In this project, inorganic and organic nitrogen water contents were monitored during flooding events at upstream and downstream locations, as well as in floodplains waters located between these 2 end-members. By comparing blackwater vs. alluvial forested floodplains and young vs. old parent material in Southeastern US, we aim to identify patterns in N removal capacities of floodplains according to the river system and the geological substrate. Other parameters such as river flow, water chemistry, and floodplain geomorphology will be also considered.

Xiaoqing Huang (Post Doctoral):

It is well known that floodplain forests can both retain and transform P, but less is known about how they accomplish these functions.  It is hypothesized that Al and Fe may operate through different pathways to reduce P loadings received in river waters during flooding events.  For Al, the reduction in pH that occurs as organic C from the floodplain surface becomes dissolved in river waters during flooding events, lowers floodwater pH, resulting in the precipitation of OM-Al-Pi complexes, removing Pi from floodwaters.  For Fe, increasing concentrations of organically-bound Fe, also driven by the increase in DOC in floodwaters, favor the formation of OM-Fe-Pi complexes, which renders Pi unavailable for uptake.  Both processes appear to be important for reducing eutrophication in downstream aquatic ecosystems through P retention or transformation.

Floodplains contribute to shaping and maintaining water quality in numerous ways. The soils within the floodplain act as filtration matrices for the flood waters retaining sedimentary material and nutrients. Significant amounts of nutrients (N, P), sediment, and organic matter originally transported by the river are deposited in floodplains during floods.

We are quantifying P retention by sediment deposition (mass, P, Al, Fe, C, and N) at each site using sediment traps (n=150/site). We have a geostatistical sampling design to accurately assess this spatially variable process and examine overlapping patterns of element deposition in relation to the processes capable of generating such patterns.