Mycorrhizal symbiosis as a strategy for survival in ultramafic soils

Ultramafic soils enriched in nickel, such as found in Australia and New Caledonia, are associated with unique, diverse and poorly known vegetation communities. Re-establishment of these highly specific ecosystems is still a challenge for Ni mining companies. Ultramafic vegetation communities are the outcome of a long evolution process resulting in their adaptation to the extreme soil conditions found on ultramafic outcrops. Mycorrhizal fungi, a very common plant symbiont, are generally thought to be beneficial to plants in other ecosystems, providing plants with phosphorus and even promoting metal tolerance in plants in some cases. We examined the hypothesis that mycorrhizal fungi may contribute to the survival of plants in ultramafic soil conditions. Bandalup Hill, an ultramafic outcrop enriched in Ni (South West of Western Australia) was selected to assess the contribution of mycorrhizal fungi to ultramafic plants. Soil constraints, in particular the degree of Ni toxicity, were assessed at two sites with ultramafic soils within the outcrop. Total metal, nutrient, DTPA extractable Ni and available P were measured in soil while Ni, Ca and Mg were tested in the soil solution. In addition, nutrients and metals were analyzed in shoots of some plant species occurring at each site: Eucalyptus flocktoniae, Melaleuca pomphostoma, Melaleuca coronicarpa and Hakea verucosa. Topsoils in Bandalup Hill and plant shoots had high levels of Ni, and very low levels of P, K and N. Variation in DTPA extractable Ni between sites reflected the variation in shoot Ni level of E. flocktoniae and M. pomphostoma. Variations in soil solution Ni levels reflected variations in shoot Ni levels of M. coronicarpa and H. verucosa between sites.