Phosphorus-efficient pastures: legume root traits for improved nutrient foraging

The critical soil phosphorus (P) requirement (P supply for 90% maximum yield) of many temperate pastures is determined by the high P requirements of key pasture legumes (commonly Trifolium or Medicago spp.). Legumes that yield well with a lower critical P requirement could reduce the fertiliser requirement of these pastures. Pot experiments were used to: (i) identify legumes with root traits likely to confer P acquisition efficiency, and (ii) test the impact of these traits on critical P requirement. An initial screen of the root hair length (RHL)
and specific root length (SRL) of 13 legumes and two grasses was undertaken. Growth and root morphology of five legumes (Trifolium subterraneum, T. hirtum, Ornithopus sativus, O. compressus, Biserrula pelecinus) and one grass (Dactylis glomerata) were subsequently compared in detail using a sandy loam soil (8.3 mg/kg Colwell P) that was amended by applying P to the top 5 cm of the soil profile (0, 15, 30, 70, 135, 250 mg P/ kg). Shoot and root growth were assessed after six weeks.
Significant variation in RHL (0.12-0.75 mm) and SRL (98-603 m/g) was found among the legumes, with most being substantially shorter (RHL) or lower (SRL) than the grasses. In the P-response experiment, Ornithopus species (the only legumes with RHL and SRL approaching that of the grasses) had critical P
requirements that were less than half that of T. subterraneum. Selecting legumes that maximise root foraging via long, thin roots with long root hairs may reduce the critical P requirement of pastures.