Nutrient limitation to nitrogen fixation in young volcanic sites

I used measures of N-15 natural abundance and of nitrogenase activity (acetylene reduction) to examine whether the supply of non-N nutrients limits rates of Nz fixation on young volcanic substrates in Hawaii. Leaves of the dominant tree (Metrosideros polymorpha, a nonfixer) were strongly depleted in N-15 in control plots (-10.8 to -11.1 parts per thousand). More than 5 y of repeated fertilization with P increased delta(15)N to -8.9 to -9.9 parts per thousand, and the addition of all other essential plant nutrients (except N) together with P further increased N-15 to -8.1 to -9.3 parts per thousand. This pattern is consistent with enhanced N-2 fixation, because newly fixed N would have a delta(15)N near 0 parts per thousand. Assays of nitrogenase activity in the experimental plots demonstrated that potential N fixation associated with nonvascular plants and with tree and fern litter were increased significantly by additions of P and by the combined nutrient treatment; when these were added together, the increase in nitrogenase activity was 6- to 11-fold over control plots. The supply of P and other weathering-derived nutrients constrains rates of N-2 fixation in these young volcanic sites and thereby contributes to the maintenance of N limitation to primary production and other ecosystem processes.