Enriching plant tissues with 13C and 15N 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 13C and 15N nuclear magnetic resonance spectroscopy. With this method, we tracked and quantified the fate of multiple pulses of 13CO₂(g) and K 15NO₃(l) in boreal tree seedling roots and leaves as a function of time. Results show that initial preferential 13C carbohydrate enrichment in the leaves was followed by redistribution to more complex compounds after seven days. While 13C allocation within the roots was uniform across molecules, 15N results indicate an initial enrichment of amine molecules after two hours.

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