Rhizosphere pH gradient controls copper availability in a strongly acidic soil.
Using a root mat approach, we quantified how root-induced alkalization controlled the establishment of copper (Cu) gradients in the rhizosphere of durum wheat (Triticum turgidum durum L.) cropped in a strongly acidic, Cu-contaminated soil. Rhizosphere pH increased over 6 mm in soil, reaching up to +2.8 units close to root mat surface. Conversely, free Cu2+ activity decreased by 3 orders of magnitude and total Cu concentration by 3-fold in the rhizosphere solution, while labile Cu assessed by DGT (diffusive gradients in thin films) was halved. The DIFS (DGT-induced flux in soils and sediments) model failed to adequately simulate Cu depletion in the rhizosphere solution, showing that root-induced alkalization almost entirely explained Cu depletion while plant uptake had little impact. We modeled the observed pH gradient to recalculate its radial extension around a single root. The gradient of free Cu2+ activity in solution, deduced from pH modeling, extended over 1-4 mm in the rhizosphere depending on root radius and OH- efflux from root. Rhizosphere alkalization dramatically decreased root exposure to Cu, substantiating that root-induced chemical changes in the rhizosphere should be better accounted for to assess metal bioavailability to plants.
Bravin
M N
MN
INRA, UMR 1222 Biogéochimie du Sol et de la Rhizosphère (INRA-SupAgro), Place Viala, 34060 Montpellier, France. matthieu.bravin@cirad.fr
Tentscher
P
P
Rose
J
J
Hinsinger
P
P
D002021:Buffers; D003300:Copper / Q000737:chemistry* / Q000378:metabolism*; D004777:Environment; D004784:Environmental Monitoring / Q000379:methods; D006863:Hydrogen-Ion Concentration; D015233:Models, Statistical; D018517:Plant Roots / Q000378:metabolism; D012044:Regression Analysis; D012987:Soil; D012989:Soil Pollutants / Q000737:chemistry*; D013997:Time Factors; D014908:Triticum / Q000378:metabolism*
D002021:Buffers; D012987:Soil; D012989:Soil Pollutants; D003300:Copper