Biological nitrogen fixation activity by co-occurring moss species is affected by their ability to absorb nitrogen from canopy throughfall.

Biological nitrogen fixation in feathermosses provides an important source of exogenous nitrogen to boreal forest ecosystems. Yet there is a limited understanding of how canopy condition and throughfall influence N fixation rates. To decipher the effect of climatic variables (i.e., temperature, humidity, and light) and nitrogen deposition chemistry (i.e., inorganic and organic N) on moss BNF, we monitored climatic variables, biological N₂-fixation, cyanobacteria biomass, and some moss traits of two co-occurring moss species in black spruce forests in Eastern Canada (Pleurozium schreberi and Ptilium crista-castrensis) over two growing seasons in undisturbed mature forests and forest gaps (windfalls). We further investigated the ability of both species to retrieve nitrogen from deposition using ¹⁵N tracers in the field and the lab. Both species achieved comparable BNF in forest gaps. In the mature forest, BNF decreased in both species compared to the forest gap but was significantly higher in P. crista-castrensis (about 10 fold) than in P. schreberi. In both species, the decrease in BNF activity between forest gaps and mature forests was likely driven by the slightly less favorable climatic conditions for BNF (e.g., temperature and light). However, BNF was further reduced in P. schreberi due to better nitrogen sorption from the throughfall leading to reduced cyanobacteria colonization and cyanobacteria-specific BNF activity. These results show that co-occurring moss species are affected in a contrasted manner by changes in the composition of nitrogen deposition.