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.

Automated summary

This study quantified the distribution of nitrogen, dry-matter, and soil-applied (15)N in different compartments of five naturally-growing oak trees. The results showed that the xylem rings compartment contained the most biomass, while branches and coarse roots contained the most nitrogen. The labeled (15)N was found in all compartments except the heartwood, with the majority being in the leaves. Overlooked compartments such as coarse roots, stumps, xylem, and other branches accounted for a significant portion of the (15)N recovery. The study also revealed that (15)N was present in all sapwood rings, with more being found in younger rings compared to older ones. The (15)N allocated to ancient rings could originate from various sources, including direct uptake from the soil, autumnal resorption from leaves, or transport through ray parenchyma. Additionally, the study confirmed the role of microbial biomass as a nitrogen sink in forests.