title: Does canopy nitrogen uptake enhance carbon sequestration by trees?
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 whether 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 nutrition, we conducted a mesocosm experiment, applying 54 g N/(15) N ha(-1)  yr(-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 also differed among treatments, with twice as much (15) N found in woody biomass when deposited on the canopy than soil. Stoichiometrically 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.
authors.affiliation: School of Geosciences, University of Edinburgh, Crew Building, Edinburgh, Midlothian, EH9 3FF, UK.
authors.forename: Richard K F
authors.initials: RK
authors.lastname: Nair
authors.affiliation: Forest Research, Northern Research Station, Roslin, Midlothian, EH25 9SY, UK.
authors.forename: Micheal P
authors.initials: MP
authors.lastname: Perks
authors.affiliation: National School of Forestry, University of Cumbria, Ambleside, LA22 9BB, UK.
authors.forename: Andrew
authors.initials: A
authors.lastname: Weatherall
authors.affiliation: Institute of Biological and Environmental Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK.
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Baggs
authors.affiliation: School of Geosciences, University of Edinburgh, Crew Building, Edinburgh, Midlothian, EH9 3FF, UK.|Institució Catalana de Recerca i Estudis Avançats, Centre for Ecological Research and Forestry Applications, Cerdanyola del Vallès, Barcelona, 08193, Spain.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Mencuccini
mesh_terms: D002244:Carbon / Q000378:metabolism; D057965:Carbon Sequestration; D009584:Nitrogen / Q000493:pharmacokinetics*; D009587:Nitrogen Isotopes / Q000493:pharmacokinetics; D028222:Picea / Q000378:metabolism*; D035261:Plant Components, Aerial / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism; D012987:Soil / Q000737:chemistry; D014197:Trees / Q000378:metabolism
chemical_list: D009587:Nitrogen Isotopes; D012987:Soil; D002244:Carbon; D009584:Nitrogen
keywords: 15N labelling; C sequestration; Nitrogen deposition; Picea sitchensis; canopy fertilization; canopy nitrogen uptake; isotope trace; soil fertilization