Boreal feather mosses secrete chemical signals to gain nitrogen

Summary The mechanistic basis of feather moss–cyanobacteria associations, a main driver of nitrogen ( N ) input into boreal forests, remains unknown. Here, we studied colonization by N ostoc sp. on two feather mosses that form these associations ( P leurozium schreberi and H ylocomium splendens ) and two acrocarpous mosses that do not ( D icranum polysetum and P olytrichum commune ). We also determined how N availability and moss reproductive stage affects colonization, and measured N transfer from cyanobacteria to mosses. The ability of mosses to induce differentiation of cyanobacterial hormogonia, and of hormogonia to then colonize mosses and re‐establish a functional symbiosis was determined through microcosm experiments, microscopy and acetylene reduction assays. Nitrogen transfer between cyanobacteria and Pleurozium schreberi was monitored by secondary ion mass spectrometry ( SIMS ). All mosses induced hormogonia differentiation but only feather mosses were subsequently colonized. Colonization on P leurozium schreberi was enhanced during the moss reproductive phase but impaired by elevated N . Transfer of N from cyanobacteria to their host moss was observed. Our results reveal that feather mosses likely secrete species‐specific chemo‐attractants when N ‐limited, which guide cyanobacteria towards them and from which they gain N . We conclude that this signalling is regulated by N demands of mosses, and serves as a control of N input into boreal forests.