Biological in situ nitrogen fixation by an Acacia species reaches optimal rates on extremely contrasted soils

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-N-2-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 N-15 abundance (delta N-15) 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 delta N-15 value of A. spirorbis was close to zero ( + 0.29 parts per thousand), while that of the reference species ranged from -4.83 to +7.05 parts per thousand. 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 delta N-15 abundance and soil parameters. Despite the difficulty of using the delta N-15 abundance method in such contrasted environments, the resulting data show that A. spirorbis has the highest N-2-fixing potential yet recorded in the genus Acacia and its nitrogen fixation is effective almost throughout its natural range.