Copper phytoavailability in vineyard topsoils as affected by pyoverdine supply

Pyoverdine (Pvd) is a bacterial siderophore produced by some Pseudomonads species that can bind copper in addition to iron in soil. Pvd is expected to alter the dynamics and the ecotoxicity of Cu in vineyard soils. This study investigated the extent to which the mobility and the phytoavailability of Cu varied among vineyard soils with different pH and how they were affected by a supply of Pvd. Pvd was supplied (or not) to ten vineyard topsoils with pH ranging from 5.9 to 8.6 before metal was extracted with 0.005 M CaCl₂. Cu mobility was assessed through its total concentration and Cu phytoavailability through its free ionic concentration measured in the CaCl₂ extract. Cu mobility varied by a factor of six and Cu phytoavailability by a factor of 5000 among the soil samples. In the CaCl₂ extract, the concentration of Cu²⁺ was not correlated with the concentration of total Cu but was correlated with pH. This revealed that Cu phytoavailability depends to a great extent on Cu complexation in soil pore water, the latter being highly sensitive to pH. Adding Pvd enhanced the mobility of Cu in the soils including in carbonate soils. The Pvd-mobilization factor for Cu varied from 1.4 to 8 among soils, linked to the availability of Fe and Al in the solid phase and to Pvd partitioning between the solid and the liquid phase. Adding Pvd reduced the concentration of Cu²⁺ in CaCl₂ extract, which challenges the idea of using Pvd-producing bacteria to promote Cu phytoextraction.