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Contrasting communities of arbuscule-forming root symbionts change external critical phosphorus requirements of some annual pasture legumes

Jeffery, Robert P., Simpson, Richard J., Lambers, Hans, Orchard, Suzanne, Kidd, Daniel R., Haling, Rebecca E., Ryan, Megan H.
Applied soil ecology 2018 v.126 pp. 88-97
Ornithopus, Trifolium subterraneum, annual pastures, cultivars, endophytes, fine roots, forage legumes, mycorrhizal fungi, phosphorus, phosphorus fertilizers, shoots, soil, symbionts
Annual pasture legumes with a superior ability to acquire soil phosphorus (P) and a low external critical P requirement could reduce the need for P fertiliser. Roots of pasture legumes grown in field soil will commonly be colonised by arbuscular mycorrhizal fungi (AMF). However, recent research suggests a second community of arbuscule-forming root-colonising fungi may be present: fine root endophytes (FRE). AMF are known to enhance P acquisition by plants under certain conditions, whereas very little is known about FRE and their impact on plant P status. We, therefore, grew plants at seven levels of P supply to determine and compare the external critical P requirements of two species of Ornithopus and two cultivars of Trifolium subterraneum when colonised predominantly by FRE (FRE-dominant treatment) or both FRE and AMF (mixed treatment). As expected from previous studies, the Ornithopus species had a lower critical P requirement than the T. subterraneum cultivars. However, for the Ornithopus species only, the FRE-dominant community had a significantly lower external critical P requirement (24–31 mg P kg−1 soil) than the mixed community (50–58 mg P kg−1 soil). Moreover, at low-P supply (≤15 mg applied P kg−1 soil), Ornithopus species in the FRE-dominant treatment had longer specific root length, smaller average root diameter, 45–128% greater total root length, and lower shoot and root P concentration than in the mixed treatment. We concluded that comparisons among experiments of root morphological traits and external critical P requirements can be affected by the presence of different communities of arbuscule-forming root-colonising fungi and that these effects may vary among plant species.