Bioavailability of zinc and phosphorus in calcareous soils as affected by citrate exudation

Abstract

Aims: Zinc (Zn) and phosphorus (P) deficiency often occurs at the same time and limits crop production in many soils. It has been suggested that citrate root exudation is a response of plants to both deficiencies. We used white lupin (Lupinus albus L.) as a model plant to clarify if citrate exuded by roots could increase the bioavailability of Zn and P in calcareous soils. Methods: White lupin was grown in nutrient solution and in two calcareous soils in a rhizobox. Rhizosphere soil solution was sampled to determine citrate, metals and P. Based on the measured citrate concentrations, a soil extraction experiment with citrate as extractant was done. Results: Absence of Zn triggered neither cluster root formation nor citrate exudation of white lupin grown in nutrient solution, whereas low P supply did. The maximum citrate concentration (~1. 5 mM) found in the cluster rhizosphere soil solution of one soil mobilized P, but not Zn. In the other soil the highest citrate concentration (~0. 5 mM) mobilized both elements. Conclusions: White lupin does not respond to low Zn bioavailability by increasing citrate exudation. Such a response was observed at low P supply only. Whether Zn and P can be mobilized by citrate is soil-dependent and the possible controlling mechanisms are discussed. © 2012 The Author(s).