Rhizosphere alkalisation — a major driver of copper bioavailability over a broad pH range in an acidic, copper-contaminated soil

The impact of a large rhizosphere alkalisation on copper (Cu) bioavailability to durum wheat (Triticum turgidum durum L.) initially exposed to a broad range of bulk soil pH (4.8-7.5) was studied. Plants were exposed to a Cu-contaminated soil treated with eight levels of lime (Ca(OH)2) and supplied with NO3- or NH4+-NO3-. Nitrate-fed plants strongly increased their rhizosphere pH to about 6.9-7.6, whatever the initial pH. NH4+-NO3--fed plants slightly acidified their rhizosphere down to 3.9. Free Cu2+ concentration in the rhizosphere was 3 orders of magnitude larger for NH4+-NO3- than NO3-fed plants. Consequently, Cu bioavailability was 2.4- to 4.2-fold larger for NH4+-NO3--fed plants which demonstrates the importance of rhizosphere alkalisation to restrict metal bioavailability in acidic soils. Copper bioavailability of NO3--fed plants initially exposed to a broad range of bulk soil pH was insensitive to bulk soil pH, as rhizosphere pH was ultimately neutral in any case.