Phosphorus limitation on nitrogen fixation by Facaltaria seedlings

The growth of nitrogen-fixing trees is often limited by the available supply of phosphorus (P) in the soil, and any factor limiting growth may also limit rates of N fixation. We examined the effect of additions of N and P fertilizer on rates of growth and N fixation of seedlings of Facaltaria moluccana (Miquel) Barneby and Grimes, using soils from 18-year-old monoculture plantations of Facaltaria or non-N-fixing Eucalyptus saligna (Sm.). No differences were found for the influence of the Facaltaria versus Eucalyptus on the soils. Seedling mass was not affected by addition of N, whereas P additions increased seedling mass by 75%. The influence of added P on N fixation was even larger, more than doubling estimates of N fixation based on N-15 pool dilution and closed-system acetylene reduction assays. A new open-system method for acetylene reduction assays correlated very well with the traditional closed system, and may offer opportunities for non-destructive, repeated sampling of N-fixation activity. (C) 2003 Elsevier B.V. All rights reserved.