Long-Term Fate Of Nitrogen Fixation In Pleurozium Schreberi Brid (Mit.) Moss Carpets In Boreal Forests

Feather mosses and associated cyanobacteria serve as an important source of nitrogen (N) in boreal forests; however, few studies have effectively traced the N2 fixation in-situ for more than one year. The purpose of this study was to assess the long-term fate of N2 fixed in feather moss carpets in boreal forests of northern Sweden. We conducted a seven-year 15N2(g) labelling and tracing experiment in a boreal forest with a moss bottom layer dominated by Pleurozium schreberi Brid (Mitt). Mesocosms of forest floor with and without mosses present were exposed to a 10% 15N2(g) atmosphere for five days after which samples of moss, shrub, pine seedling and O horizon were taken at varied intervals and analysed for delta 15N. Plots were split and burned in year seven to assess the effect of fire on liberating moss N for uptake by Scots pine (Pinus sylvestris L.) seedlings. Feather mosses treated with 15N2(g) were highly enriched in 15N one month after labelling which subsequently declined after seven years to 13% of the original 15N enrichment in green tissue. No measurable 15N enrichment was observed in shrubs or the O horizon in the presence of labelled mosses until greater than five years after the labelling experiment. Fire can liberate moss N, but we did not observe a direct effect of fire on N uptake by pine seedlings. This long-term study indicates that N2 fixed by cyanobacteria in feather moss carpets is conserved in moss tissue for extended periods and slowly transferred to the forest soil O horizon as moss tissue decomposes suggesting that moss N2 fixation is most important as an ecosystem N source at long time scales.