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

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.