Nodulation and growth of subterranean clover ( Trifolium subterraneum L.) in a drying acid soil

Abstract

Subterranean clover (cv. Mt Barker) was grown in undisturbed acid soil (pH 4.3), in cultivated acid soil (pH 4.3) and in lime-amended (pH 5.5) soil and after 12 weeks of growth was either watered to field capacity or basally irrigated with the equivalent of half the amount of water transpired by the control plants. Basally-irrigated plants were able to exploit subsoil moisture and maintain adequate plant-water relations whilst the soil at the top of the soil profile dried slowly. Most nodules were confined to the less acidic uppermost and basal regions of the undisturbed acid soil profile and no new nodules were produced on the root system once basal irrigation commenced. The permanent wilting point of the compact, undisturbed acid soil (−1.5 MPa) corresponded with a soil moisture content of 7% w/w and was attained 3 weeks after commencement of basal irrigation. Nitrogen accumulation by the subterranean clover plants grown in this treatment ceased when the top 9 cm of the acid soil profile reached the permanent wilting point. The permanent wilting point of the top 9 cm of the limed and unlimed cultivated soil occurred at a soil moisture content of 5% w/w and was reached 5 weeks after commencement of basal irrigation. Nodules occurred on roots at depth in the limed and unlimed cultivated soil and nodulation continued after commencement of basal irrigation compensating for lower nodule numbers on the upper crown roots that were in contact with the drying soil. Nitrogen continued to be accumulated by these plants throughout the experiment. Nodule size varied in accordance with soil cultivation treatment. In particular, larger nodules were produced on the roots of plants grown in undisturbed acid soil whereas smaller, more numerous nodules occurred on the roots of plants grown in lime-amended soil and to a lesser extent on the roots of plants grown in cultivated acid soil. However, total nodule mass was equivalent on plants grown in each of the soil treatments. Continued nodulation at depth on the roots of subterranean clover due to cultivation and incorporation of lime enabled nitrogen fixation to continue when nodules on roots near the soil surface became desiccated and senescent due to the localized moisture deficit.