Nitrogen fixation of Acacia mangium Willd. from two seed sources grown at different levels of phosphorus in an Ultisol, South Sumatra, Indonesia

Phosphorus (P) is required to facilitate the fixation of atmospheric nitrogen (N) by leguminous species such as Acacia mangium. We studied the N fixation of A. mangium trees grown from two seed sources. These consisted of bulk seedlots collected from seed orchards in Sumatra, one based on natural provenances from the Cairns region, Queensland, Australia, and the other on the Muting natural provenance, Papua, Indonesia. The seedlots were grown under contrasting P supply (application rates of 0 and 100 kg ha⁻¹) on acidic clay soils in a field experiment at Lematang, South Sumatra. The ¹⁵N natural abundance method was used, with Eucalyptus pellita as a non-N-fixing reference species. Foliage samples were collected at age 12 and 18 months. Application of P increased stem volume, foliage N concentrations and the amount of N fixed in both seed sources at 12 and 18 months of age. For the Cairns-origin seed source, the percentage of N derived from atmospheric N was low (c. 14%) without P addition, but increased by more than four-fold with P addition. In contrast, N fixed by the Muting-origin seed source (c. 45-60%) was not significantly affected by P addition. The Cairns-origin seed source partly compensated for lower N fixation by taking up a larger amount of soil N. Thus there were significant differences between seed sources in their N-fixing capacity and the effect of P nutrition on this process. The higher productivity of Muting under low P application appeared to be associated with better P utilisation and greater N fixation, but it was still responsive to P application.