Factors affecting the early growth of Leucaena leucocephala .2. Importance of arbuscular mycorrhizal fungi, grass competition and phosphorus application on yield and nodulation of leucaena in pots

Slow seedling growth is a Limitation to the more widespread adoption of the tree legume, leucaena (Leucaena leucocephala). Three glasshouse trials examined the role of arbuscular mycorrhiza and phosphorus (P) nutrition in determining early growth and nodulation of leucaena. Treatments included soil types, inoculation with arbuscular mycorrhizal (AM) fungi, P application, grass competition and fumigation with methyl bromide, an anti-fungal agent. Plant measurements included colonisation by AM fungi, nodule weight, tissue nitrogen (N) and P concentrations. Slower early growth of leucaena in a soil from Mt Cotton than in soils from Gayndah or Theodore was due to slow colonisation of roots by AM fungi. Sequential harvests of plants revealed that rate of colonisation in the Mt Cotton soil was only half that in the Theodore soil prior to 28 days after planting resulting in subcritical P concentrations 21 days after sowing and an approximate halving of top dry weight 41 days after sowing. However, following increased infection, tissue P concentration and final plant growth 98 days after sowing were similar in both soils. Early seedling growth in the Mt Cotton soil was increased by inoculating the soil with mulch containing AM fungi but not with soil collected from beneath established leucaena added at a lower rate. Phosphorus application significantly increased growth of leucaena seedlings, but only the highest rate of 1200 kg P/ha was able to prevent early P deficiency. Final growth was reduced by 50% in the presence of Panicum maximum as a result of increased competition for N and P and by 90% in fumigated soil as a result of P deficiency. The results of these experiments confirm the important role of AM fungi on early seedling growth of leucaena. However, the potential to increase early growth using a soil or mulch inoculum containing AM fungi or P fertiliser may be limited by the high rates of application needed. More work is needed to determine whether slow rate of infection is a significant limitation in soils other than the Mt Cotton soil in the field.