Feather moss nitrogen acquisition across natural fertility gradients in boreal forests

Feather mosses utilize various sources of nitrogen (N): they absorb N deposited on leaf tissue, they host N2 fixing cyanobacteria, and they are able to take up N directly from soil. In addition to their importance as primary producers in boreal ecosystems, feather mosses play a significant role in N cycling. However, estimates of their ability to take up N from soil in situ are scarce. Further, connecting uptake of N from soil with N2 fixation could significantly improve our understanding of their role in ecosystem N cycling, but to date this issue has not been addressed. We report results from an uptake experiment in which we tracked 13C-carbon (C), 15N-alanine and 15N-ammonium chloride (NH4Cl) into feather moss (Pleurozium schreberi (Brid.) Mitt.)-soil cores taken along natural fertility gradients in Northern Sweden. The varying fertility conditions coincided with a N2 fixation gradient in the feather moss. We found that P. schreberi takes up C and N directly from soil. However, the moss did not show a preference for inorganic or organic N sources and only 1.4% of the added amino acid appeared to be taken up from soil in an intact form. No differences in uptake of C or N from soil along the fertility gradients were detected. Nitrogen fixation rates in the moss were thus not correlated with C or N-uptake from soil. Nitrogen fixation as well as uptake of C and N from soil seem to be unaffected by C or N availability in the soil, suggesting that the moss can cover its nutrient demand by absorption of throughfall N and via associated N2-fixing cyanobacteria without soil-N supplementation. We suggest further, that the moss can represent a (temporary) N-sink in the boreal forest, and that the moss' mechanism of uptake and release thereby will characterize the ecosystem N cycle.