Arbuscular mycorrhizal fungi and sewage sludge enhance growth and adaptation of Metrosideros laurifolia on ultramafic soil in New Caledonia: A field experiment.

The ecological restauration of nickel mining-degraded areas in New Caledonia is strongly limited by soil mineral nutrient deficiencies, heavy metal toxicity and slow growth rate of the native plant species. The improvement of revegetation technics needs multidisciplinary knowledge. A field experiment with relocated topsoil was assessed to test plant inoculation with a mix of three selected arbuscular mycorrhizal fungi (AMF) combined with sewage sludge amendment of the soil. Metrosideros laurifolia seedlings, an endemic Myrtaceae, were inoculated with the mixed AMF isolates and grown in a nursery for 18 months before being planted. 528 days after plantation, the dry weight of inoculated plants was 4 times higher than non-inoculated ones. AMF inoculated plants growing in sewage sludge amended soil showed a dry weight more than seven times higher than control plants. These differences were positively correlated with mycorrhizal colonization. However at this stage, AMF inoculated isolates were reduced in roots of M. laurifolia and replaced by several AMF indigenous species. This AMF diversity was higher in inoculated plants and in non-amended plots. Inoculated plants were characterized by a better mineral nutrition, a higher Ca/Mg ratio and a lower heavy metal translocation. In conclusion, this study showed that AMF inoculation combined with sewage sludge amendment of soil can improve ecological restoration of ultramafic mine-degraded areas.