Does canopy nitrogen uptake enhance carbon sequestration by trees?

Richard K. F. Nair, Micheal P. Perks, Andrew Weatherall, Elizabeth M. Baggs, Maurizio Mencuccini

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Abstract

Temperate forest (15) N isotope trace experiments find nitrogen (N) addition driven carbon (C) uptake is modest as little additional N is acquired by trees, However, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked if N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply (15) N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional (15) N additions typically trace mineral (15) N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutition, we conducted a mesocosm experiment, applying 54 g N /(15) N ha(-1) y(-1) to Sitka spruce saplings. We compared tree and soil (15) N recovery among treatments where enrichment was due to either 1) a (15) N-enriched litter layer, or mineral (15) N additions to 2) the soil or 3) the canopy. We found that 60 % of (15) N applied to the canopy was recovered above ground (in needles, stem, and branches) while only 21 % of (15) N applied to the soil was found in these pools. (15) N recovery from litter was low and highly variable. (15) N partitioning among biomass pools and age classes and also differed among treatments, with twice as much (15) N found in woody biomass when deposited on the canopy than soil. Stochiometrically calculated N effect on C uptake from (15) N applied to the soil, scaled to real-world conditions, was 43 kg C kg N(-1) , similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N(-1) . Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

Original languageEnglish
Pages (from-to)875-888
Number of pages14
JournalGlobal Change Biology
Volume22
Issue number2
Early online date14 Dec 2015
DOIs
Publication statusPublished - Feb 2016

Bibliographical note

Acknowledgements

We thank Archie Crofton and Radina Atanasova (University of Edinburgh), as well as Colin McEvoy and Carina Convery (Forest Research, UK) for field and technical assistance. We would like to thank Bill Mason (Forest Research UK) for access to the Gisburn Forest tree species trial. Isotope analyses were performed by Nick Morley at the University of Aberdeen, UK. This work was funded by the UK Natural Environment Research Council (NERC), grant NE/G00725 X/1 and Forestry Commission, UK. Funding to pay the open access cost of this publication was provided by NERC.

Keywords

  • nitrogen deposition
  • Picea sitchensis
  • 15N labelling
  • isotope trace
  • canopy fertilization
  • soil fertilization
  • canopy nitrogen uptake
  • C sequestration

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