Quantifying nitrogen-fixation in feather moss carpets of boreal forests

Biological nitrogen (N) fixation is the primary source of N within natural ecosystems1, yet the origin of boreal forest N has remained elusive. The boreal forests of Eurasia and North America lack any significant, widespread symbiotic N-fixing plants1,2,3,4,5,6. With the exception of scattered stands of alder in early primary successional forests7, N-fixation in boreal forests is considered to be extremely limited. Nitrogen-fixation in northern European boreal forests has been estimated2 at only 0.5 kg N ha-1 yr-1; however, organic N is accumulated in these ecosystems at a rate of 3 kg N ha-1 yr-1 (ref. 8). Our limited understanding of the origin of boreal N is unacceptable given the extent of the boreal forest region, but predictable given our imperfect knowledge of N-fixation1,9. Herein we report on a N-fixing symbiosis between a cyanobacterium (Nostoc sp.) and the ubiquitous feather moss, Pleurozium schreberi (Bird) Mitt. that alone fixes between 1.5 and 2.0 kg N ha-1 yr-1 in mid- to late-successional forests of northern Scandinavia and Finland. Previous efforts have probably underestimated N-fixation potential in boreal forests.