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 N-15 (delta(15)N)-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 delta(15)N of NO3- in soil solution generally is isotopically lighter than the delta(15)N 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 delta(15)N-values in their tissues than species growing equally well, or better, on NH4+. We studied the delta(15)N of six 4 understory species in oak woodlands in southern Sweden at 12 sites which varied fivefold in potential net N mineralisation rate (minNH(4)(+) + 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 minNO(3)(-) and leaf delta(15)N 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 delta(15)N-value. Furthermore, delta(15)N 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 delta(15)N 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 5 15 N needs careful consideration of known restrictions of method, soils and plants. (C) 2004 Elsevier Ltd. All rights reserved.