Acid soil infertility and associated toxicities of aluminum and manganese are barriers to the reforestation of former sugarcane lands in Hawaii. The purpose of the study is to facilitate reforestation of acid soils through the combined use of acid-toerant genotypes of Acacia koa, arbuscular mycorrhizal fungi, and bradyrhizobia.

OBJECTIVES

1) Screen Acacia koa provenances for tolerance to soil acidity.
2) Isolate bradyrhizobial species nodulating A. koa and arbuscular mycorrhizal fungi from diverse Hawaiian soils that are known to have pH values of 4.0 to 5.5.
3) Characterize isolates obtained under objective 2 with respect to ability to rapidly colonize and express symbiotic effectiveness in A. koa in acid soils containing different concentrations of aluminum and manganese ions.
4) Evaluate the efficacy of superior isolates of Bradyrhizobia and AM fungi for ability to enhance biomass production by A. koa in acid soils containing toxic levels of aluminum and manganese ions in the field.

APPROACH

We will collect seeds of Acacia koa from trees growing in soils characterized by low pH in various parts of the Hawaiian islands. We will identify provenances of Acacia koa tolerant to aluminum and manganese by planting the seeds in acid medium containing phytotoxic levels of aluminum and manganese. We will concurrently collect soil samples from these areas for isolation and identification of aluminum- and manganese-tolerant arbuscular mycorrhizal fungi and bradyrhizobia. The isolates will be characterized for their symbiotic efficacy in soil containing phytotoxic levels of aluminum and manganese ions. Promising A. koa provenances will be combined with highly effective acid tolerant arbuscular mycorrhizal and bradyrhizobial isolates in order to enable the legume to establish and grow on acid lands without liming and fertiization with nitrogen and phosphorus.

Of the arbuscular mycorrhizal isolates we obtained, one isolated from Al-rich acid soil and another one obtained form Mn-rich acid soil exhibited relatively high inoculum potential. These were evaluated for their effect in enhancing nutrient uptake and growth of A. Koa in Mn-rich and Al-rich acid soils, respectively. Bradyrhizobia isolated from the Al-rich soil induced relatively high levels of AM fungal colonization on A. Koa roots at pH 4.5. The level of colonization was not only significantly higher than that observed in the acid soil not inoculated with the isolates but also was comparable to that observed at pH 5.7. Similarly, the isolate obtained from the Mn-rich acid soil increased AM fungal colonization of A. Koa roots significantly and the level of colonization observed was similar or better than that observed at pH 5.9. Leaf P status was significantly increased due to inoculation in both systems. However, the growth of A. koa was not stimulated significantly due to inoculation by any of the isolates in either of the soils. However, plants grown in the inoculated soils defoliated to a lesser extent and were appreciably greener than those grown in the non-inoculated soil. The results suggest that either the AM fungi did not have sufficient time to enhance the growth of A. koa or that some other factor was limiting the growth of the legume. It is possibile that the symbiotic effectiveness of the isolates was very low.

It is promising that we could isolate AM fungi that were able to colonize root surfaces of A. koa to the extent they did in the presence of toxic levels of aluminum or manganese. We now have to identify those isolates that are highly infective as well as symbiotically effective. This will have significant impact on the establishment of A. koa in acid soils containing high level of aluminum or manganese.

PUBLICATIONS (not previously reported): 2005/09 TO 2006/09

M. Habte. 2006. Contributions of arbuscular mycorrhizal fungi to soil and plant health in the tropics. A paper presented at the Permanent Seminar of the National University of Colombia at Medellin, Medellin, Colombia.