Quantifying nitrogen-fixation in feather moss carpets of boreal forests

Abstract

Biological nitrogen (N) fixation is the primary source of N within natural ecosystems, 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 plants. With the exception of scattered stands of alder in early primary successional forests, N-fixation in boreal forests is considered to be extremely limited. Nitrogen-fixation in northern European boreal forests has been estimated 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-fixation. 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.

Symbiosis revisited: phosphorus and acid buffering stimulate N<sub>2</sub> fixation but not <i>Sphagnum</i> growth

“…This is on the same order of magnitude as the range of 12-25 kg ha −1 y −1 reported for pristine boreal bogs, although their growing season only lasts 140 days per year (Vile et al, 2014). Furthermore, similar to Markham (2009), we found Sphagnum-associated N 2 fixation rates to be at least 5 times higher than those found in feather mosses, which are around 1.5-3 kg ha −1 yr −1 (Rousk et al, 2014;DeLuca et al, 2002;Zackrisson et al, 2009;Leppänen et al, 2013). This could be due to morphological differences between the moss species (including hyaline cells of Sphagnum providing additional space and protection to microorganisms) and differences in microbial communities resulting from differences in habitat conditions and resources, i.e., availability of inorganic and organic nitrogen and carbon compounds, moisture content and presence of oxygen.…”

Variation in N2Fixation in Subarctic Tundra in Relation to Landscape Position and Nitrogen Pools and Fluxes

“…However, a wide span of values ranging between 0.56 and 22 has been reported in studies from terrestrial ecosystems (summarized in Liengen, 1999a). Studies on N 2 fixation in northern ecosystems reported ratios 0.022-4.9 (Liengen, 1999a;DeLuca et al, 2002;Sorensen et al, 2006). The reason for the relatively high conversion factor in our soils can be explained by low nitrogenase activity, but remains inconclusive.…”

“…Many studies also documented an association of Nostoc sp. with Pleurozium schreberi (DeLuca et al, 2002;Houle et al, 2006;Gundale et al, 2012). However, soil cores covered with this moss species were the only ones from vegetated PP that did not exhibit any nitrogenase activity in our study.…”

Uptake and recovery of soil nitrogen by bryophytes and vascular plants in an alpine meadow

“…Generally, bryophytes are very efficient in assimilating N, which are competitive scavengers of N and can reduce N availability of vascular plants, because bryophytes can not only absorb N from soil directly but can also fix atmospheric N 2 by forming facultative symbioses with cyanobionts (DeLuca et al 2002;Turetsky 2003;Ayres et al 2006). In addition, bryophytes absorb more ammonium-N than nitrate-N and glycine-N.…”