Bryophytes attenuate anthropogenic nitrogen inputs in boreal forests

Productivity in boreal ecosystems is primarily limited by available soil nitrogen (N), and there is substantial interest in understanding whether deposition of anthropogenically derived reactive nitrogen (N-r) results in greater N availability to woody vegetation, which could result in greater carbon (C) sequestration. One factor that may limit the acquisition of N-r by woody plants is the presence of bryophytes, which are a significant C and N pool, and a location where associative cyanobacterial N-fixation occurs. Using a replicated stand-scale N-addition experiment (five levels: 0, 3, 6, 12, and 50 kg N ha(-1)yr(-1); n=6) in the boreal zone of northern Sweden, we tested the hypothesis that sequestration of Nr into bryophyte tissues, and downregulation of N-fixation would attenuate N-r inputs, and thereby limit anthropogenic N-r acquisition by woody plants. Our data showed that N-fixation per unit moss mass and per unit area sharply decreased with increasing N addition. Additionally, the tissue N concentrations of Pleuorzium schreberi increased and its biomass decreased with increasing N addition. This response to increasing N addition caused the P. schreberi N pool to be stable at all but the highest N addition rate, where it significantly decreased. The combined effects of changed N-fixation and P. schreberi biomass N accounted for 56.7% of cumulative N-r additions at the lowest N-r addition rate, but only a minor fraction for all other treatments. This 'bryophyte effect' can in part explain why soil inorganic N availability and acquisition by woody plants (indicated by their delta N-15 signatures) remained unchanged up to N addition rates of 12 kg ha(-1)yr(-1) or greater. Finally, we demonstrate that approximately 71.8% of the boreal forest experiences N-r deposition rates at or below 3 kg ha(-1)yr(-1), suggesting that bryophytes likely limit woody plant acquisition of ambient anthropogenic Nr inputs throughout a majority of the boreal forest.