Bryophyte-cyanobacterial biogeochemistry: Limited evidence of nitrogenase activity in northern-Wisconsin boreal bryophytes

Background/Question/Methods   Bryophyte-cyanobacterial symbioses are an important source of biologically-fixed nitrogen in circumpolar boreal forests, contributing over half of the fixed nitrogen to some far-northern ecosystems. Environmental nitrogen availability is known to influence whether bryophyte-cyanobacterial associations occur, as well as rates of nitrogen fixation by moss-dwelling cyanobacteria. Specifically, low nitrogen availability has been correlated with both the formation of symbioses and with higher rates of N fixation by the cyanobacterial symbionts. Most studies to date have focused on far-northern ecosystems. In this study the frequency and rates of bryophyte-cyanobacterial N fixation are examined in a boreal forest in northern Wisconsin. The overall goal of this study is to answer the following questions: (1) Are the bryophyte mats in a temperate-latitude boreal habitat a significant source of fixed nitrogen, as they are in the boreal forests farther north, and (2) if so, what factors influence the incidence and rate of fixation?

Results/Conclusions   During 2009, the acetylene-reduction assay was used to estimate the frequency and rate of nitrogen fixation in bryophyte samples across a 1-km gradient perpendicular to the shore of Lake Michigan, including 9 boreal forest sites and 1 beach site. A total of 1140 random bryophyte samples were subjected to the acetylene-reduction assay during 10 different weeks (114 samples/week), ranging from early May to late October. Data were collected on potential environmental controls such as bryophyte hydration and substrate, vapor pressure and temperature of site, and relative age and soil N of site. However, none of the boreal sites demonstrated measurable nitrogenase activity at any point during the study, making the influence of potential control factors difficult to assess. By contrast, the semi-aquatic beach site did exhibit consistently high indirect measures of nitrogen fixation, with 98% of over 250 samples positive for nitrogenase activity. Overall, the results indicate that bryophyte-cyanobacterial associations are not necessarily important elements of nitrogen cycling in every ecosystem, even when moss is abundant. Further research is needed to clarify the controls and impacts of bryophyte-cyanobacterial nitrogen fixation, particularly the global extent of such activity.