Bryophytes attenuate anthropogenic nitrogen inputs in boreal forests

• Published in: Global change biology Vol. 17; no. 8; pp. 2743 - 2753
• Main Authors: GUNDALE, MICHAEL J, DELUCA, THOMAS H, NORDIN, ANNIKA
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01-08-2011; Wiley-Blackwell
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; biological nitrogen fixation; Biomass; boreal forests; Bryophyta; bryophytes; Bryopsida; carbon; Carbon sequestration; carbon sink; Climate change; cyanobacteria; ecosystems; Forest Science; Forests; Fundamental and applied biological sciences. Psychology; General aspects; mosses and liverworts; Nitrogen; nitrogen content; nitrogen deposition; nitrogen fixation; Plant cytology, morphology, systematics, chorology and evolution; Pleurozium schreberi; productivity; Skogsvetenskap; soil; soil nitrogen uptake; Terrestrial ecosystems; woody plants; Sweden; Productivity; Boreal forest; Atmospheric fallout; nitrogen deposition; Bryophyta; soil nitrogen uptake; biological nitrogen fixation; Nitrogen; Carbon; Carbon sequestration; bryophytes; Soils; carbon sink; Cyanobacteria; Nitrogen fixation; Bacteria; Pleurozium schreberi; Anthropogenic factor
• ISSN: 1354-1013; 1365-2486; 1365-2486
• DOI: 10.1111/j.1365-2486.2011.02407.x

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 (Nr) results in greater N availability to woody vegetation, which could result in greater carbon (C) sequestration. One factor that may limit the acquisition of Nr 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 Nr inputs, and thereby limit anthropogenic Nr 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 Nr additions at the lowest Nr 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 δ¹⁵N 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 Nr 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.