Plant nitrate use in deciduous woodland: the relationship between leaf N, N natural abundance of forbs and soil N mineralisation

Our aim was to study whether the in situ natural abundance ¹⁵N (δ¹⁵N)-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 and or to alone. Several studies show that the δ¹⁵N of in soil solution generally is isotopically lighter than the δ¹⁵N of due to fractionation during nitrification. Hence, it is reasonable to assume that plant species benefiting from in ecosystems without significant leaching or denitrification have lower δ¹⁵N-values in their tissues than species growing equally well, or better, on We studied the δ¹⁵N of six understory species in oak woodlands in southern Sweden at 12 sites which varied fivefold in potential net N mineralisation rate The species decreased in benefit from 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 and leaf δ¹⁵N and a positive correlation between and leaf N concentration. In wide contrast, only D. flexuosa, which grows on soils with little nitrification, showed a positive correlation between and the leaf N concentration and δ¹⁵N-value. Furthermore, δ¹⁵N of plants from the field and previously obtained indices of hydroponic growth on relative to were closely correlated at the species level. We conclude that δ¹⁵N may serve as a comparative index of uptake of among understory species, preferably in combination with other indices of N availability. The use of δ¹⁵N needs careful consideration of known restrictions of method, soils and plants.