Abstract
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 (δ15N) 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 δ15N 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 δ15N abundance and soil parameters.Despite the difficulty of using the δ15N 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.
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