Our aim was to study whether the in situ natural abundance ¹⁵N (δ ¹⁵N)-values and N concentration of understory plants were correlated with the form and amount of mineral N available in the soil. Also to determine whether such differences were related to earlier demonstrations of differences in biomass increase in the same species exposed to nutrient solutions with both NO3- and NH4+ or to NH4+ alone. Several studies show that the δ ¹⁵N of NO3- in soil solution generally is isotopically lighter than the δ ¹⁵N of NH4+ due to fractionation during nitrification. Hence, it is reasonable to assume that plant species benefiting from NO3- in ecosystems without significant NO3- leaching or denitrification have lower δ ¹⁵N-values in their tissues than species growing equally well, or better, on NH4+. We studied the δ ¹⁵N of six understory species in oak woodlands in southern Sweden at 12 sites which varied fivefold in potential net N mineralisation rate (minNH4++NO3-). The species decreased in benefit from NO3- in the following order: Geum urbanum, Aegopodium podagraria, Milium effusum, Convallaria majalis, Deschampsia flexuosa and Poa nemoralis. Four or five species demonstrated a negative correlation between minNO3- and leaf δ ¹⁵N and a positive correlation between minNO3- and leaf N concentration. In wide contrast, only D. flexuosa, which grows on soils with little nitrification, showed a positive correlation between minNH4+ and the leaf N concentration and δ ¹⁵N-value. Furthermore, δ ¹⁵N of plants from the field and previously obtained indices of hydroponic growth on NH4+ relative to NH4++NO3- were closely correlated at the species level. We conclude that δ ¹⁵N may serve as a comparative index of uptake of NO3- among understory species, preferably in combination with other indices of N availability. The use of δ ¹⁵N needs careful consideration of known restrictions of method, soils and plants.