Acacia spirorbis subsp. spirorbis Labill. is a legume tree that grows on calcareous, metalliferous and siliceous soils and is endemic to New Caledonia. The present study assesses the nitrogen-fixing potential of A. spirorbis in its contrasted natural environments. Soil samples, the leaves of A. spirorbis and of co-occurring non-N2-fixing reference plant species were collected from nine study sites across New Caledonia. Soil properties were analyzed by ICP-AES. Nitrogen (N) content and natural 15N abundance (d15N) were measured in A. spirorbis and in reference plants. The percentage of N derived from the atmosphere (%Ndfa) fixed by A. spirorbis was also assessed and correlated with soil parameters. Remarkably, mean N contents in A. spirorbis showed no significant differences whatever the soil categories (20.2¿±¿3.5¿g¿kg-1) whereas major differences were found in N contents in the reference plants. The average d15N value of A. spirorbis was close to zero (+0.29‰), while that of the reference species ranged from -4.83 to +7.05‰. In eight of the nine sites studied, A. spirorbis %Ndfa was above 70%, regardless of the soil category. Unlike in the reference species, no statistical correlations were found between A. spirorbis N content and d15N abundance and soil parameters. Despite the difficulty of using the d15N abundance method in such contrasted environments, the resulting data show that A. spirorbis has the highest N2-fixing potential yet recorded in the genus Acacia and its nitrogen fixation is effective almost throughout its natural range.