Professor Middlebury College Middlebury, VT, United States
Abstract: 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. These nitrogen-fixing bacteria can live freely within soils or in symbiosis with plants. In recent years, it has been determined that bryophytes support these microbial communities epiphytically and endophytically, explaining a missing link in the nitrogen balances of moss-dominated systems. Moss-cyanobacteria relationships exist from the tropics to the tundra. In particular, they have been explored in boreal forests and boreal peatlands contributing significantly to nitrogen inputs, and, as a result, carbon sequestration, in these regions. However, moss-cyanobacteria symbioses and their ecosystem impacts remain largely unexamined in temperate peatlands.
Using coupled microscopy and N15 enriched incubations, we were able to establish the presence of cyanobacteria colonies across species of Sphagnum moss and quantify nitrogen fixation in three temperate peatlands, ultimately building a predictive model for nitrogen fixation in these systems as driven by biological and environmental factors. Data collection was distributed along a latitudinal gradient from Massachusetts to Northern Vermont over the course of the 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 holistic approach to sampling revealed correlations between abiotic factors (temperature, humidity, and precipitation) and the rate of moss-associated nitrogen fixation, highlighting the control of more immediate weather conditions on the performance of cyanobacteria and presenting distinct niches of certain species when and where nitrogen fixation is optimized. In the face of anthropogenic climate change, where effects are not anticipated to occur uniformly but as a trend punctuated by extreme temperatures and novel precipitation regimes of drought and deluge, predictive models like the one we built have potential to estimate the consequences of different climate models and better understand the future of nitrogen cycling and tightly coupled carbon cycles.
