NODULATION BY FRANKIA INCREASES GROWTH OF CASUARINACEAE IN A NEW-ZEALAND HORTICULTURAL SOIL

Casuarina cunninghamiana and C. glauca are widely used for shelterbelt s in New Zealand and have other utilitarian potential. Their nitrogen (N)-fixing Frankia symbiont does not occur naturally in New Zealand so ils. Plants nodulated after artificial inoculation with Frankia cultur e HFPCcI3 were planted on a horticultural-quality Manawatu silt loam s oil near Palmerston North. After 6-8 years, trees nodulated at plantin g were significantly larger in stem diameter and height (equivalent to 16-61% additional volume) over trees not nodulated at planting. Nodul ated trees were also rated higher for vigour. Excavation of root syste ms revealed that nodules were confined to the residual zone of potting media around the original seedling root systems and there was no evid ence of Frankia spreading through the soil to infect nearby roots. Mor e widespread availability of Frankia inoculum in the soil being explor ed by feeder roots might have increased nodulation and subsequent tree growth, but New Zealand soil temperatures are suboptimal for Casuarin aceae nodulation and N fixation.