Influence of plants and cropping on microbiological characteristics of some New Caledonian ultramafic soils

Five New Caledonian ultramafic soils were compared for their bacterial and fungal population densities and for their microbial activity estimated by fluorescein diacetate (FDA) hydrolysis. The low microbial numbers and activities were related to the organic matter content and to metallic toxicity. Actinomycetes were found to be dominant among bacterial populations. The effect of soil colonisation by plants on the microflora was studied and appeared to be very favourable. The rhizosphere effect of 2 plant species (Grevillea gillivrayi and Costularia comosa) was estimated. The influence of cropping on the development of microflora in one soil was also investigated and a qualitative study of the fungal populations and their variation in relation to the cropping was reported. The fungal flora was dominated by Moniliaceae, and Tuberculariaceae and Dematiaceae were absent in natural ultramafic soils. Cropping enhanced the diversity of these fungal populations.