Moss-associated nitrogen fixation across scales: from shoot to landscape

Abstract Biological nitrogen fixation (BNF) performed by moss-associated colonizing cyanobacteria is among the main sources of new nitrogen (N) inputs in pristine subarctic ecosystems. Moisture is the primary driver of BNF in cyanobacteria associated with mosses. Yet, knowledge of BNF and its potential drivers on smaller spatial scales is lacking. To address these knowledge gaps, we measured BNF along vertical layers (segments) representing different depths and age along moss shoots of two dominant species, Pleurozium schreberi and Hylocomium splendens over 6 weeks. Mosses were collected along a steep precipitation gradient in subarctic Northern Sweden, and factors known to affect BNF like contents of iron (Fe), phosphorus (P), nitrogen (N) and molybdenum (Mo), as well as pH were analysed. BNF capacity varied significantly and profoundly along the moss shoots and the precipitation gradient, and was highest in H. splendens and in mosses collected at the high end of the precipitation gradient. Contents of Fe, Mo and N generally increased with segment depth while P concentrations decreased and pH was stable across segments. While differences in BNF along the precipitation gradient were clear and largely explained by the importance of moisture for moss-associated BNF, nutrient content also seemed to affect BNF in mosses, although less pronounced. P. schreberi showed highest BNF in the middle segment, while BNF in H. splendens was highest in the lowest segments. Differences in cyanobacterial communities and different nutrient requirements may explain species specific BNF responses. This should be considered when aiming to upscale BNF to ecosystem level.