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

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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.