Interactions between lucerne, rhizobia and mycorrhizas under different levels of N and P in the glasshouse and field

Abstract

Use of nitrogen (N) fixing legumes and mycorrhizal inoculants have the potential to reduce the effects of limiting soil N and phosphorus (P) levels in agricultural soils. In New Zealand, lucerne is used for direct grazing and hay making and it is recommended that rhizobia (Ensifer meliloti) and P be added to this crop for maximum production. The specific objectives of this thesis were to test under different soil N and P availability: 1) The effectiveness of Ensifer meliloti rhizobial inoculum and four strains of Rhizobium sp. isolated from lucerne in New Zealand on growth of lucerne. 2) The effectiveness of commercial vesicular arbuscular mycorrhizas on growth of lucerne. 3) The interaction between rhizobial and mycorrhizal inoculant on growth of lucerne. In pots, Ensifer meliloti increased, but Rhizobium sp. isolated from New Zealand soils decreased, lucerne total plant dry matter. It seems likely that under field conditions, competition between Ensifer meliloti in the inoculum and less effective indigenous soil rhizobial strains for nodulation can reduce the efficiency of the inoculum. Addition of N and P with Ensifer meliloti both increased lucerne total plant dry matter but shoot crude protein, dry matter digestibility and metabolisable energy were not affected. Plants relying solely on N2 fixation had around 90% total dry matter of plants on optimum soil N indicating that there is little benefits of adding N to lucerne if it is adequately nodulated. Addition of P is required in low P soils to achieve high production.

Under field conditions, addition of mycorrhizal inoculum or 16 kg P/ha gave similar increased dry matter yield but addition of rhizobial inoculant did not affect yield. It seems likely that there were already high populations of rhizobia in the soil before rhizobial inoculation as plants sampled from uninoculated plots showed substantial nodulation. Thus, the effect of additional rhizobia was negated. Overall, similar yield increases of lucerne with mycorrhizas and added 16 kg P/ha is an important finding. The potential of mycorrhizas as a mechanism to reduce P inputs into lucerne crops warrants further testing under different soils and agricultural systems in New Zealand.