Drivers of copper and zinc availability and phytoavailability in agricultural soils receiving long-term organic waste amendments
Résumé
Organic wastes (OW) are used as soil amendments and fertilizers but they are also
the major source of copper (Cu) and zinc (Zn) contamination in agricultural soils.
The potential ecotoxicological effects of this contamination on soil organisms
depend on Cu and Zn availability in soils. The availability of Cu and Zn itself
depends on their chemical speciation and consequently to the temporal evolution of
soil parameters such as pH and organic carbon content and reactivity. These soil
parameters are key parameters both influenced by the application of OW and the
activity of soil organisms on the surrounding soil, i.e. the rhizosphere for plants.
However, the concomitant influence of OW applications and rhizosphere effect are
poorly documented when taking into account long-term impacts. Accordingly, we
aimed at studying the relationship between the availability in soil and the
phytoavailability of Cu and Zn in four decadal field trials that received different
types of OW for more than ten years. Soils in the four field trials exhibited very
different pH and organic carbon content. Copper and Zn availability was
determined on 102 soil samples from the four field trials by (i) an equilibrium-based
method using cupric ion selective electrode and the windermere humic aqueous
model (WHAM) to quantify Cu2+ and Zn2+ activities in soil solutions (pCu2+ and
pZn2+) and (ii) a kinetic method using the diffusive gradient in thin films (DGT)
directly on soil samples. We measured key soil parameters in soil solutions to assess
the relationship with pCu2+ and pZn2+. Copper and Zn phytoavailability is currently
determined using the RHIZOtest which is a standardized biotest that will enable to
measure the uptake flux of Cu and Zn in plants and the related availability of Cu and
Zn in the rhizosphere that is physically separated from roots. The results already
achieved showed no clear relationship between pCu2+ and pH or dissolved organic
carbon among the four field trials altogether. When studying each trial separately,
we observed a pH gradient as a function of the type of fertilizer (mineral or OW)
applied, which influence substantially the speciation of Cu in soil. The on-going
measurements will enable to test whether Cu and Zn availability in unplanted soils
determined Cu and Zn phytoavailability or whether root-induced chemical changes
in the rhizosphere additionally determined it.