Scott D. Bridgham

Nutrient Dynamics in Wetlands
           

“Multiple Environmental Gradients Structuring Peatland Communities” (National Science Foundation CAREER award DEB9629415), $420,000, Sept. 1996 – Aug. 2003. 

Principal Investigators:  Scott D. Bridgham

Collaborator:  John Pastor, University of Minnesota, Duluth

Graduate Students:  Carmen Chapin (National Park Service), Laurie Kellogg (Oklahoma State University), Colleen Iversen (University of Tennessee), Jason Keller (Smithsonian Environmental Research Center)

Technicians:  Kathleen Lysyshyn, Peter Weishampel (USDA Forest Service), Karen Updegraff (South Dakota State University)

            We undertook a number of experiments under this grant, and have done related research under other grants, on nutrient dynamics in wetlands.  Much of this research has focused on the role of nutrients in structuring peatland ecosystems along a ombrotrophic-minerotrophic gradient (Bridgham et al. 1995, 1996, 1998, 2000, 2001).  Ombrotrophic peatlands, or bogs, receive all their water and associated elemental inputs from precipitation and, therefore, are acidic and are assumed to have very low nutrient availability.  Their dominant vegetation(Sphagnum mosses, ericaceous shrubs, and black spruce) is thought to reflect extreme nutrient limitation.  In contrast, minerotrophic peatlands, or fens, receive water from ground- and/or surface-water, and thus have a higher pH, are thought to have high nutrient availability, and are vegetated by graminoids. While the ombrotrophic-minerotrophic gradient is fundamentally defined by hydrology (and related basic cation inputs), it has been assumed traditionally to be coincident with an oligotrophic-eutrophic gradient of nutrient availability.  We have shown that N availability does increase with increasing minerotrophy, but that this is due to a much larger soil nitrogen pool, as the small soil nitrogen pool in bogs turns over rapidly (Updegraff et al. 1995, Bridgham et al. 1998, 2001).  Nitrification and nitrate availability are also greater in more minerotrophic peatlands (Bridgham et al. 1998, 2001).  The small total phosphorus pool in bogs also turns over rapidly, and phosphorus availability is often similar or greater in bogs compared to fens (Bridgham et al. 1998, 2001, Chapin 1998, Kellogg and Bridgham, 2003, Chapin et al. 2003).  Different plant species are typically N and/or P limited, but this varies by functional group and plant species within functional groups (Bridgham et al. 1995, 2001, Chapin 1998, Chapin et al. 2004, Iversen 2005, Keller et al. 2006).  Sphagnum mosses have a very low tolerance for N fertilization and only moderate levels of N addition are toxic (Chapin et al. 2004, Keller et al. 2006).

            To better understand the role of nutrients in structuring plant communities in peatlands, we initiated a fertilization and liming experiment in a bog and fen in Minnesota.  This experiment consisted of 20 3-m2 plots at each site, with each plot partially hydrologically isolated by rubber roofing material buried to ~30-cm depth. One set of plots at each site was fertilized with 2 or 6 g N m-2 yr-1 and 0.67 or 2 g P m-2 yr-1 in four different treatment combinations.  A second set of plots at each site received the nutrient treatments plus sufficient lime to raise the pH of the bog (3.8) to that of the fen (4.9) and the pH of the fen to that of a ‘rich’ fen (6.4).  We also established a lime only and a control treatment.  Each treatment was replicated twice.  This experiment constituted the dissertation research of Carmen Chapin, and she followed changes in plant community composition, aboveground productivity, and N and P mineralization dynamics from 1995-1998 (Chapin 1998, Chapin et al. 2003, 2004).  We continued these treatments through 2002.

            A second large fertilization experiment was initiated at the University of Notre Dame Environmental Research Center (UNDERC) in northern Michigan by Laurie Kellogg for her dissertation research.  She added 6 g N m-2 yr-1 and 2 g P m-2 yr-1 to three bogs, three intermediate fens, and two rich fens from 1998 – 2002.  She applied isotopic tracers (32P and 15N) in the field to each of these wetlands and examined the distribution of phosphorus and nitrogen through the various abiotic and biotic pools through time (Kellogg et al. 2003, in preparation).  She also compared previously published methods of determining in situ rates of gross phosphorus mineralization and devised a preferred method of performing this important, but rarely made measurement (Kellogg et al. 2006).

            As part of this experiment, Colleen Iversen examined nutrient use efficiency at hierarchical levels of ecological organization (from the leaf to the whole-ecosystem) for her M.S. (manuscript in preparation).

            We also have examined the more theoretical foundations of nutrient use efficiency in two papers (Bridgham et al. 1995, Pastor et al. 1999).

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