Competitive Mobilization of Phosphate and Arsenate Associated with Goethite by Root Activity

Arsenate (AsV) is the predominant form of arsenic in soils under aerobic conditions and competes with the major plant nutrient phosphorus (P) in the form of phosphate (PV) not only for sorption sites on mineral surfaces in soil but also for root membrane transporters. Plants have evolved several mechanisms for the mobilization of PV in soils in response to P deficiency, such as the release of organic anions and protons. The aim of the present study was to test whether these mechanisms result in a simultaneous mobilization of arsenate and what would be the consequences for As transfer from soil to plant. The compartment system approach with Zea mays as model crop was chosen as an experimental setup. The system is equipped with micro suction cups and allowed us to investigate processes occurring in the vicinity of roots. As a case study, an artificial quartz substrate with well defined soil physical properties was fertilized, spiked with AsV, and amended with increasing amounts of goethite (0, 1, and 4 g kg-1 in treatments G-0, G-1, and G-4, respectively). The addition of goethite alleviated the AsV-induced growth reduction and reduced AsV transfer from the substrate to the plant but induced P deficiency at the same time. When low amounts of goethite (1 g kg-1) were added, plants mobilized PV but not AsV, which might be related to differences in surface complexation reported for PV and AsV. No mobilization of PV or AsV was observed with the addition of higher amounts of goethite, probably because of decreasing competition between organic anions, PV, and AsV for binding sites.