Plant communities in forests are influenced by the availability and form of nitrogen in the soil, however, often little is known about the depth of uptake or species of N taken up by plants in diverse communities. In order to assess the relative influences of rooting and uptake depth, plant storage of N, and form of N taken up, we compared the natural abundance 15N of roots and shoots of 65 species of understory plants to the 15N of those same species given a source of 15N-enriched litter.

The study was conducted at 5 beech stands which are part of the French long-term forest monitoring program RENECOFOR. Natural litter was replaced with 15N-enriched litter for a period of 18 months prior to sampling and analysis of 15N of plants in the treated plots. The mean enrichment in 15N

(Δδ15N) was calculated by subtracting the natural abundance δ15N of the species (from the control plot) from the mean δ15N value of the same species in the labelled plots.Results/Conclusions

While δ15N of herbaceous plants and ligneous plants were comparable (from-9 to -1‰ for herbaceous and  -8 to -2‰ for ligneous), the Δδ15N  of ligneous plants was always low compared to the mean Δ15N of herbaceous plants. For example at one site, ligneous plant Δ15N  ranged from 2-9 ‰ and herbaceous plant Δ15N  ranged from 1-26‰., indicating that ligneous plants used proportionately less of the 15N labelled N released in the upper forest floor. At four of the five sites, natural abundance δ15N was inversely related to Δδ15N values in herbaceous plants, suggesting that shallow-rooted with low initial δ15N had greater access to the tracer 15N released in the upper forest floor. Highest Δ15N and lowest δ15N were fournd for plants without share perennial organs. At the other end of the continuum, ancient forest species, that often have storage, had higher initial δ15N and low Δδ15N values, which suggest they are poor competitors for N. As expected, N-fixing plants had circumneutral initial δ15N and Δδ15N values. This gradient was net seen as the fifth site, where N deposition was about twice as high as at the other sites. The results suggest that  forest understory species, as afunction of Raunkier’s life forms, utilize sources of N that vary in depth and form.