Does canopy nitrogen uptake enhance carbon sequestration by trees?

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.