Arbuscular mycorrhizae in wheat and field pea crops on a low P soil: increased Zn-uptake but no increase in P-uptake or yield

Few field studies have investigated the contribution of arbuscular mycorrhizal fungi (AMF) to agricultural systems. In this study, the role of AMF in nutrition and yield of dryland autumn-sown wheat and field pea was examined through a 2-year crop sequence experiment on a red loam (Kandosol) in SE Australia. The soil was P-deficient and had low levels of root pathogens. In Year 1, levels of AMF were increased by growing subterranean clover or Linola™ and decreased by growing canola or through maintenance of bare fallow with herbicides or tillage. In Year 2, hosts of AMF (wheat and field pea) and non-mycorrhizal canola were grown with 0 P or 20 kg ha-1 of P as superphosphate. Yields of all Year 2 crops were increased by P-fertiliser. Year 1 treatment led to 2–3 fold variation in colonisation by AMF at each P-level for Year 2 wheat and field pea. High colonisation did not correspond with greater crop growth, yield, or uptake of P, K, Ca, Cu or S in wheat or field pea. However, total crop Zn-uptake and grain Zn concentration were positively correlated with colonisation by AMF, due to enhanced Zn-uptake after anthesis. For wheat, high colonisation also corresponded with reduced Mn-uptake and lower grain Mn concentrations. In a glasshouse experiment using a second P-deficient Kandosol, inoculation of wheat with Glomus intraradices and Scutellospora calospora enhanced uptake of Zn and P when no P-fertiliser was applied. We conclude that high colonisation by AMF is unimportant for productivity of the major field crops grown on the Kandosol soils that occupy large areas of cropland in temperate SE Australia, even under P-limiting conditions. Investigation of the factors that control functioning of arbuscular mycorrhizae under field conditions, especially temperature, is required.