Eurasian steppe ecosystems are nitrogen-limited and suffer additionally from high grazing intensities in many areas. Soil surface-bound cyanobacteria are able to fix nitrogen and can be the major source of plant available nitrogen in such ecosystems. In this study, the abundance and dinitrogen fixation capacity of the most common soil surface-bound microbial and lichen species were determined at an ungrazed, a winter-grazed, and a heavily grazed steppe site in the Xilin River catchment, Inner Mongolia, People’s Republic of China. The microorganisms were identified as Nostoc spec. and the lichen species as Xanthoparmelia camtschadalis (Ach.) Hale by a combination of classical light microscopy, confocal laser scanning microscopy and molecular analysis of the internal transcribed spacer (ITS1) region of ribosomal RNA. Both species were found exclusively at grazed steppe sites, with a clear difference in abundance depending on the grazing intensity. At the winter-grazed site, Nostoc was more abundant than Xanthoparmelia; for the heavily grazed site, the opposite was found. N2 fixation was quantified with both the acetylene reduction method and 15N2 incubation. Cyanobacterial colonies of Nostoc fixed N2 vigorously, whereas X. camtschadalis did not at all. The fraction of nitrogen derived from the fixation of molecular nitrogen in Nostoc was 73%, calculated from 15N natural abundance measurements of Nostoc with X. camtschadalis as reference. The conservatively calculated N2 uptake by Nostoc was 0.030–0.033 kg N ha−1 for the heavily grazed site and 0.080–0.087 kg N ha−1 for the winter-grazed site for the growing seasons of 2004 and 2005, respectively. Together with previous findings, this study demonstrates that N2 fixation by Nostoc can potentially replace significant amounts, if not all, of the nitrogen lost in the form of N2O and NO soil emissions in this steppe ecosystem.
