A MODEL SYSTEM FOR ASSESSING LOW-LEVEL LIMING EFFECTS ON WHITE CLOVER SYMBIOSIS DEVELOPMENT IN AN ACIDIC SOIL

Abstract

Establishment and maintenance of forage legumes on acidic soils continues to be a worldwide problem. In an attempt to develop a more rational approach to the selection of plants and bacteria, a laboratory model system was constructed using an acidic, fresh Ultisol from an old-field. The objective of the study was to evaluate this system by determining the effect of low-level liming of the soil to pHw 4.62, 4.76, 4.83, and 4.95, and rhizobial inoculation, on the establishment and functioning of the Trifolium repens, L./Rhizobium leguminosarum bv. trifolii symbiosis. The soil, as taken from the field, was low in pHw (4.65) and base saturation (45%), relatively high in Al (2.4 cmol kg1), and essentially devoid of naturalized rhizobia. For the symbiotically-grown (inoculated) plants, beneficial effects of liming to only pHw 4.83 on shoot mass, shoot N uptake, nodule number, and ARA, but not root mass and length, were detected as early as 3 weeks. Nearlinear responses to soil pH increases were found for shoot mass, shoot N uptake, nodule number, and acetylene reduction activity at 5 and, with the exception of ARA, 7 weeks. Root mass, but not length, also responded positively to liming at these later times. Soil pH and basic cation- and aluminum-associated properties were significantly (P ≤ 0.05) correlated with shoot mass and shoot N uptake. Although correlations (r) improved with time, they never exceeded 0.80, even at 7 weeks. Asymbiotically grown (NH4NO3-treated) plants gave similar shoot responses, but at different times, indicating dissimilar growth rate responses compared with the symbiotically grown plants. Rhizobial inoculum levels altered the outcome of the symbiosis significantly, indicative of a rhizobial survival limitation. Taken together, these results suggest that both partners of the symbiosis, plants and rhizobia, were affected adversely by the stressful chemical composition of the soil. More importantly, these results demonstrate the utility of the model system for studying the early (3 weeks or less) effects of low-level liming on the development of the white clover symbiosis in a near-natural, acidic Ultisol.