Sphagnum-associated Nitrogen Fixation in New England Peatlands

Life is inextricably linked to nitrogen, yet most nitrogen exists in a biologically unavailable form, gaseous N2. N2 is converted into NH4+, readily taken up by plants and microbes, through nitrogen fixation as facilitated by diazotrophic bacteria, notably cyanobacteria. Using coupled microscopy and field-based 15N2 incubations, we examined cyanobacteria colonization across species of Sphagnum moss and quantified associated nitrogen fixation in three temperate peatlands. Data collection was distributed along a latitudinal gradient from Massachusetts to Northern Vermont over the course of a growing season (June-September) and included intra-peatland dynamics reflective of bog heterogeneity including microtopography and changing ecozones across transects from the center to edge of each bog. This sampling approach revealed species specific rates of cyanobacterial colonization and correlations between abiotic factors (temperature, humidity, light and precipitation) and the rate of moss-associated nitrogen fixation, highlighting the control of short-term weather conditions on the nitrogen fixing performance of cyanobacteria associating with Sphagnum. In all, Sphagnum-associated nitrogen fixation was estimated to be between 1,200 - 1,600 mg N m-2 year-1, depending on site, revealing that the rate of biological nitrogen fixation in temperate peatlands is around the same as rates within boreal peatlands, and roughly equivalent to regional rates of atmospheric nitrogen deposition.