Abstract
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.
Highlights
Interest in the role of the nitrogen (N) cycle in ongoing global change has driven a large number of studies into the effects of N deposition and the dynamics of N pools within ecosystems (e.g., Nadelhoffer et al 1999b, Magill et al 2004, Magnani et al 2007)
Nitrogen's stable isotope, 15N, is often used as an enriched tracer in spikes of mineral 15N additions (e.g., Högberg 1997, Nadelhoffer et al 1999a, Mulholland and Tank 2000, Templer et al 2012), or at natural abundance (e.g., Högberg 1990, Dijkstra et al 2008), to investigate N dynamics beyond that which can be measured in bulk changes in pools and fluxes
The 21 g l−1 solution took between 2 and 10 days to reach the threshold estimated concentration of 1 g l−1, and uptake times (mean 6.4 ± 2.3 (SD) days) displayed by individual trees were not related to total tree mass (P > 0.05), needle mass (P > 0.05) or canopy ratio (P > 0.05)
Summary
Interest in the role of the nitrogen (N) cycle in ongoing global change has driven a large number of studies into the effects of N deposition and the dynamics of N pools within ecosystems (e.g., Nadelhoffer et al 1999b, Magill et al 2004, Magnani et al 2007). Biomass enriched in 15N can be produced by application of labelled fertilizers (Weatherall et al 2006, Langenbruch et al 2013), foliar sprays (Zeller et al 1998) or by direct injections into the plant vascular system (Swanston and Myrold 1998). This latter methodology is potentially most efficient as the valuable 15N-labelled material is not lost via misting (Bowden et al 1989), exposed to soil sinks (Nadelhoffer et al 1999b) or exported from the immediate area by soil hydrology. Injection techniques (Roach 1939) were first utilized to apply enriched
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