Phosphate and arsenate interactions in the rhizosphere of canola (Brassica napus)

Arsenate [As(V)] and phosphate [P(V)] compete for the same uptake systems in plant roots. For this reason, P(V) is often added to As-contaminated soils to reduce As(V) uptake and alleviate As toxicity. However, the addition of P(V) can result in an increase in As concentrations in plant tissues, which is often attributed to an increase in As(V) availability, although direct evidence for this explanation is lacking. Rhizosphere and batch desorption experiments were conducted to investigate (i) the effect of As(V) and P(V) additions, and plant phosphorus (P) status, on As(V) uptake by canola (Brassica napus L.) and (ii), P(V) and As(V) interactions at the surface of roots and specific soil particles (goethite and kaolinite). Results showed that P-deficient canola took up more As(V) from arsenated kaolinite, but transported less As from roots to shoots, than P-adequate canola. The addition of P(V) adsorbed on substrate increased As(V) uptake by canola but desorption experiments revealed that the addition of phosphated kaolinite to arsenated kaolinite was not likely to increase As(V) availability in the rhizosphere. It was concluded that plant P status together with P(V) and As(V) interactions at the surfaces of roots and soil particles need to be considered to properly asses P(V) and As(V) interactions in the plant–soil continuum.