Plant nitrate use in deciduous woodland: the relationship between leaf N, 15N 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 NO 3 − and NH 4 + or to NH 4 + alone. Several studies show that the δ15N of NO 3 − in soil solution generally is isotopically lighter than the δ15N of NH 4 + due to fractionation during nitrification. Hence, it is reasonable to assume that plant species benefiting from NO 3 − in ecosystems without significant NO 3 − leaching or denitrification have lower δ15N-values in their tissues than species growing equally well, or better, on NH 4 + . 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 ( minNH 4 + + NO 3 − ) . The species decreased in benefit from NO 3 − 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 minNO 3 − and leaf δ15N and a positive correlation between minNO 3 − and leaf N concentration. In wide contrast, only D. flexuosa, which grows on soils with little nitrification, showed a positive correlation between minNH 4 + and the leaf N concentration and δ15N-value. Furthermore, δ15N of plants from the field and previously obtained indices of hydroponic growth on NH 4 + relative to NH 4 + + NO 3 − were closely correlated at the species level. We conclude that δ15N may serve as a comparative index of uptake of NO 3 − 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.