Phosphorus supply alters the root metabolism of Chinese flowering cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsenet Lee) and the mobilization of Cd bound to lepidocrocite in soil

Soil P status affects plant root exudates, thereby influencing heavy metal mobilization. We analyzed the effect of soil P forms and levels on lepidocrocite-bound Cd mobilization by Chinese flowering cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee). Sand pot culture experiments were performed. Sufficient and low level water-soluble P simulated sufficient and deficient P fertilizer supply, respectively. Using FePO4 and AlPO4, we simulated sparingly soluble P species in subtropical red soil. P deficiency and sparingly soluble P reduced the volume of Chinese flowering cabbage root exudates. Organic acids were the major metabolites under all treatments. Amino acid metabolism diminished under low P and Al-P. Organic acid secretion occurred in the following order: sufficient P > Fe-P > low P > Al-P. These organic acids could release lepiodcrocite-bound Cd. However, excess PO43- could precipitate Cd under sufficient P. The total mobilized Cd in rhizosphere was Fe-P > low P > Al-P = sufficient P. P promoted root metabolite secretion in Chinese flowering cabbage, especially organic acids. P deficiency facilitated the mobilization of lepidocrocite-bound Cd by organic acids. When phosphorus was sufficient, the precipitation of phosphate on Cd was greater than the mobilization of organic acids.