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Research article2011Peer reviewed

Bryophytes attenuate anthropogenic nitrogen inputs in boreal forests

Gundale, Michael; DeLuca, Thomas H.; Nordin, Annika

Abstract

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.

Keywords

biological nitrogen fixation; bryophytes; carbon sequestration; carbon sink; cyanobacteria; nitrogen deposition; Pleurozium schreberi; productivity; soil nitrogen uptake

Published in

Global Change Biology
2011, Volume: 17, number: 8, pages: 2743-2753
Publisher: WILEY-BLACKWELL