PS 10-100
Plant-microbial associations and n fixation: Sensitivity to changing moisture levels in a Lake Michigan beach-ridge system

Monday, August 5, 2013
Exhibit Hall B, Minneapolis Convention Center
Shana L. Ederer, Botany, University of Wisconsin at Madison, Madison, WI
Sara C. Hotchkiss, Department of Botany, University of Wisconsin, Madison, WI
Background/Question/Methods

Plant-microbial associations are a potentially critical source of organic nitrogen in many ecosystems.  Among the plants that associate with N-fixing bacteria, the bryophytes (mosses, liverworts, and hornworts) may respond with unusual speed to environmental change, given the lack of water-conservation mechanisms (e.g., roots, stomata on leaves, multistratose tissues) characteristic of vascular plants.  In this experiment, bryophyte groundcover in a beach-ridge ecosystem in Bailey's Harbor, Wisconsin was used to test the following hypotheses: (1) Bryophytes of wetland swales will harbor active N-fixing bacteria more often than bryophytes of adjacent dune ridges; and (2) Lack of water on short time scales (e.g., within a given day) limits the activities of N-fixing bacteria associated with beach-ridge coastal bryophytes. These hypotheses on water limitations were tested in paired upland/wetland ridge and swale sites at five areas in the beach-ridge system.  The acetylene reduction assay (ARA), an indirect measure of nitrogen-fixing activity in biological systems, was applied to 300 bryophyte samples in the summer of 2011.  Paired samples were subjected to ambient moisture and to saturated conditions. A mixed-effects model was applied to the binomial ARA response data (0 = absence of N-fixing activity, 1 = presence of N-fixing activity); predictors included two fixed-effects terms (wetland status and water treatment) and two random-effects terms (site nested within sampling date).  The diversity of bryophytes associated with N-fixing cyanobacteria was also surveyed. 

Results/Conclusions

Both wetland status and the water addition treatment had significant impacts on N fixation activity (p-values 2.83 x 10-5 and 0.0304, respectively).  Nitrogen fixation was more likely to be detected in wetland areas than in uplands, and saturated samples were more likely to be associated with N fixation than samples with ambient moisture levels.  Distinctive assemblages of mosses were associated with the wetland and upland areas. Ceratodon purpureus, Brachythecium oxycladon, and Tortula ruralis were N-fixers in the dry, sandy uplands; Drepanocladus aduncus, Drepanocladus verniscosus, Brachythecium turgidum, Brachythecium oxycladon, Bryum pseudotriquetrum, Campylium stellatum, Campylium pogonatum, Thuidium delicatulum, Fissidens adianthiodes, and Sphagnum sp. were N-fixers in wetland areas.  These results indicate that changes in water availability in coastal beach-ridge systems, particularly wetlands, have the potential to change both assemblages of bryophytes and the activity of N-fixers they host.  As climate change modifies moisture availability in Great Lakes coastal wetlands, biotic patterns of N fixation may change substantially.