Recently absorbed nitrogen incorporates into new and old tissues: evidence from a 15 N-labelling experiment in deciduous oaks

We quantified the distribution of nitrogen (N), dry-matter (biomass) and of soil-applied 15 N in tree and soil compartments in five naturally-growing 20-year-old oak trees. After applying 15 N solution to soil at the base of the trees in spring, all the trees were felled in the fall, their root system excavated, biomass, nitrogen and 15 N content measured in all compartments. Xylem rings-compartment contains most biomass (47%) while branches and coarse-root contains most nitrogen (29% and 14% respectively). The labelled 15 N absorbed throughout the vegetation season, was found in all compartments except the heartwood. The majority of recovered 15 N was in the leaves (24%). Some often overlooked compartments (coarse root, stump, xylem and other branches) together recovered 45% of the 15 N. 15 N was found in all the sapwood rings, from the ring formed in the current year up to 10 year-old rings, marking the limit of the heartwood. More 15 N was found in the younger rings compared to older rings. The 15 N allocated to ancient rings can originate from different, non-mutually exclusive, sources: whether directly from the soil via the 15 N uptake throughout the vegetation season and transport in the xylem sap, from the autumnal resorption of 15 N first allocated to the leaves, or from the 15 N mobility once allocated to the forming ring to older rings through ray parenchyma. With about 6% of the initial 15 N retrieved in the microbial biomass at the end of the growing season, we confirmed the role of microbial biomass as forest nitrogen sink.