Stem injection of 15N–NH4NO3 into mature Sitka spruce (Picea sitchensis)

Stem injection techniques can be used to introduce 15N into trees to overcome a low variation in natural abundance and label biomass with a distinct 15N signature, but have tended to target small and young trees, of a variety of species, with little replication. We injected 98 atom% 15N ammonium nitrate (NH4NO3) solution into 13 mature, 9- to 13-m tall edge-profile Sitka spruce trees in order to produce a large quantity of labelled litter, examining the distribution of the isotope throughout the canopy after felling in terms of both total abundance of 15N and relative distribution of the isotope throughout individual trees. Using a simple mass balance of the canopy alone, based on observed total needle biomass and modelled branch biomass, all of the isotope injected was accounted for, evenly split between needles and branches, but with a high degree of variability both within individual trees, and among trees. Both 15N abundance and relative within-canopy distribution were biased towards the upper and middle crown in foliage. Recovery of the label in branches was much more variable than in needles, possibly due to differences in nitrogen allocation for both growth and storage, which differ seasonally between foliage and woody biomass.