Tracking Stable Isotope Enrichment in Tree Seedlings with Solid-State NMR Spectroscopy

Enriching plant tissues with (13)C and (15)N isotopes has provided long-lasting, non-reactive tracers to quantify rates of terrestrial elemental fluxes (e.g., soil organic matter decomposition). However, the molecular location and level of isotope enrichment may differ among plant tissues. This factor is central to the integrity and interpretation of tracer data, but is seldom considered in experiments. We propose a rapid, non-destructive method to quantify molecular isotope allocation using solid-state (13)C and (15)N nuclear magnetic resonance spectroscopy. With this method, we tracked and quantified the fate of multiple pulses of (13)CO(2)(g) and K (15)NO(3)(l) in boreal tree seedling roots and leaves as a function of time. Results show that initial preferential (13)C carbohydrate enrichment in the leaves was followed by redistribution to more complex compounds after seven days. While (13)C allocation within the roots was uniform across molecules, (15)N results indicate an initial enrichment of amine molecules after two hours.