Nodulation of Casuarinaceae in Relation to Host Species and Soil Properties

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Abstract
A field survey was conducted in Australia to examine nodulation of Casuarinaceae in relation to 22 host species (187 sites) and soil chemical properties (152 sites). Four of the five Casuarina species examined were regularly nodulated (most plants in the 60 out of 88 sites in which nodules were found). Casuarina species occurred more frequently on soils of higher available-phosphorus status than did Allocasuarina species, eight of which did not nodulate in any soils. With the nine Allocasuarina species which nodulated,nodules occurred on only a minority of the individual plants examined. Soil chemical properties other than available-phosphorus level were useful in distinguishing sites at which individual species occurred but had no apparent relationship to nodulation. Under glasshouse conditions, baiting of field soils with seedlings of Casuarinaceae indicated the occurrence of nodulation to be similar to that observed in the field survey. Allocasuarina species formed ectomycorrhizas more commonly than did Casuarina species; both genera formed vesicular-arbuscular mycorrhizas. A second glasshouse experiment supported the hypothesis that low phosphorus supply and the absence of infective Frankia were two of the factors responsible for the absence of nodulation in some field soils. The effects of other soil factors in limiting plant growth and nodulation of Casuarinaceae are considered, and the potential significance of N2 fixation by Casuarinaceae in the field discussed. Nodules were found on Allocasuarina campestris, A. dielsrana and A. lehmanniana, not previously recorded as nodulating.