Phosphorous uptake by grain legumes and subsequently grown wheat at different levels of residual phosphorous fertiliser

A considerable portion of the phosphorus ( P) fertilisers applied in agriculture remains in the soil as sorbed P in the forms of various P compounds, termed residual P. Certain grain legume crops may be able to mobilise residual P through root exudates, and thus increase their own growth, and potentially that of subsequent cereal crops. The first objective of this pot experiment was to compare the growth and P uptake of 3 legume crop species with that of wheat grown in a soil with different levels of residual P. Another objective was to determine whether the influence of legumes on subsequent P uptake by wheat was due to legume-induced changes in the rhizosphere, or to the presence of legume roots. White lupin ( Lupinus albus L.), field pea ( Pisum sativum L.), faba bean ( Vicia faba L.), and wheat ( Triticum aestivum L.) were grown in a soil containing 25.7, 26.4, 30.8, 39.0, or 51.9 mg/kg of bicarbonate-extractable P and sufficient amounts of nitrogen to suppress nodulation and dinitrogen fixation. Differences among the species in root dry mass were much larger than those in shoot dry mass. Faba bean produced the greatest root dry mass. All the legumes exuded carboxylates from their roots, predominantly malate, at all soil P levels. Rhizosphere concentrations of carboxylates were highest for white lupin, followed by field pea and faba bean. All of the investigated legumes enhanced the growth of the subsequently grown wheat, compared with wheat grown after wheat, even at relatively high levels of soil P. The positive effect on growth was not dependent on the incorporation of the legume roots into the soil. The legumes also caused a modest increase in wheat shoot P concentrations, which were higher when roots were incorporated into the soil. Because of the increased growth and tissue P concentrations, wheat shoot P content was 30 - 50% higher when grown after legumes than when grown after wheat. The study concludes that the legume crops can enhance P uptake of subsequently grown wheat, even at relatively high levels of residual P.