Peltigera, a genus of dinitrogen-fixing, terricolous lichens: Its influence on soil processes in northern forests of Minnesota

Dinitrogen-fixing lichens (cyanolichens) are a well-recognized source of exogenous nitrogen (N), yet earth-dwelling (terricolous) cyanolichens are little studied. Cyanolichens of the genus Peltigera occur on or near soil in diverse habitats. They are often the dominant terricolous cyanolichen, yet no studies of their influence on soil processes are reported. The thesis of my research is that Peltigera contributes N through leaching and decay and thereby provides nutrients for soil microorganisms in northern forests. Hypotheses are (1) Peltigera leaks N in situ; (2) soil N increases near Peltigera; and (3) early-phase decomposition increases near Peltigera.; (1) Leachate from Peltigera and non-dinitrogen-fixing lichens was collected for 49 days in northeastern Minnesota and analyzed for N. Forests of northern Minnesota were surveyed to estimate Peltigera cover. We sketched peltigerous thalli at 104 sites in 256 3m x 50m quadrats. Sketches were digitized and cover determined. (2) Ion-exchange resin was buried for one growing season along 150cm transects radiating up- and downslope from Peltigera in contrasting habitats, then extracted. Soil was collected along 150cm transects originating at Peltigera and along control transects, laboratory-incubated for 74 weeks under optimal microbial conditions, and periodically extracted. Extracts were analyzed for inorganic N. Soil water and temperature were measured along transects originating at Peltigera. Soil N was measured 10cm and 120cm from Peltigera in forests across northern Minnesota. (3) Mass and nutrient loss were measured in Acer saccharum and Pinus strobus litter decomposed 12 months near Peltigera or moss. Peltigera leachate was N-enriched (p < 0.05), whereas rain and leachate from non-dinitrogen-fixers were similar. Nitrogen leached from Peltigera is estimated at 193 kg per year in northeastern Minnesota forests (2 million hectares). Current peltigerous biomass contains about 81.6 Mg N. The soil investigations have demonstrated an association of Peltigera with significantly increased available N, mineralizable N, percent N, and mass and N loss from litter (p-values < 0.05). Data suggest that soil water and temperature are not primary factors. These studies indicate a potential zone of influence, or thallosphere, extending 150cm from Peltigera. Thus, peltigerous lichens in N-limited northern forests are likely of ecological significance to soil microorganisms.