Nitrogen fixation by subterranean clover ( Trifolium subterraneum L.) in an acid soil in response to moisture deficits

Abstract

Subterranean clover cv. Mt Barker was grown in intact cores of (i) undisturbed acid soil (pH 4.3), (ii) cultivated acid soil (pH 4.3) and (iii) lime-amended soil (pH 5.5) for 12 weeks after which half of the plants were watered from the base with a sub-optimal amount of water. Normal plant-water relations were maintained by plants in basally-irrigated treatments although the soil at the top of the profile began to slowly dry. Nodules in the drying soil also began to slowly lose moisture. Loss of moisture from nodules was very rapid once the permanent wilting point of the soil (−1.5 MPa) had been reached. Large nodules produced on the roots of plants grown in undisturbed acid soil recorded a water potential (ψ nod) of −1.3 MPa after the soil had reached the permanent wilting point whereas the smaller nodules produced on plants grown in lime-amended soil were more severely desiccated (ψ nod of −1.8 MPa) after the permanent wilting point had been reached. Most of the root nodules occurred in the top 9 cm of the soil cores and acetylene reduction activity of nodulated roots in this region of the soil profile (divided into 0–4 and 4–9 cm segments) was measured in a flow-through system. Moisture deficits reduced the acetylene reduction activity of nodules. In moist soil, nitrogenase activity was similar for plants in all treatments. However, irrespective of nodule size, highest rates of nitrogenase activity occurred at a ψ nod > −0.4 MPa (RWC nod of 82%), and acetylene reduction activity declined rapidly when ψ nod fell below this level.