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

^a UFZ Helmholtz Centre for Environmental Research, Dep. of Soil Ecology, Theodor-Lieser-Str. 4, 06120 Halle, Germany
^b Soil Science and Soil Protection Group, Inst. of Agriculture and Nutritional Sciences, Martin Luther Univ. Halle-Wittenberg, Weidenplan 14, 06108 Halle/Saale, Germany
^c UFZ Helmholtz Centre for Environmental Research, Dep. of Analytical Chemistry, Permoserstr. 15, 04318 Leipzig, Germany
^d UFZ Helmholtz Centre for Environmental Research, Department of Soil Chemistry, Theodor-Lieser-Str. 4, 06120 Halle, Germany

View larger version (10K): [in this window] [in a new window]   Fig. 1. Change of initial arsenate (AsV) and phosphate (PV) concentration and their molar ratio in soil solution with increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1). Soil solution was sampled after allowing 6 d for equilibration. Values given represent the mean of all suction cup positions within each treatment.

View larger version (27K): [in this window] [in a new window]   Fig. 2. Effect of increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on soil solution arsenate (AsV) concentration at increasing distance from the center of the root compartment with time. Horizontal hair lines at 16 mm distance from the center of the root compartment represent the location of the nylon net (root surface). Soil solution was sampled 10, 17, 24, and 31 d after planting (DAP).

View larger version (27K): [in this window] [in a new window]   Fig. 3. Effect of increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on soil solution phosphate (PV) concentration at increasing distance from the center of the root compartment with time. Horizontal hair lines at 16 mm distance from the center of the root compartment represent the location of the nylon net (root surface). Soil solution was sampled 10, 17, 24, and 31 d after planting (DAP). *No sample was available.

View larger version (15K): [in this window] [in a new window]   Fig. 4. Temporal pattern of molar phosphate (PV):arsenate (AsV) ratio in the soil solution of the root compartment with increasing amounts of goethite application to the artificial substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1). In treatment G-4, the ratio could only be calculated for 3 and 10 d after planting; thereafter, PV concentrations were close to zero. The bars, representing SD, refer to the variation between the five suction cups positioned in the root compartment.

View larger version (15K): [in this window] [in a new window]   Fig. 5. Effect of increasing amounts of goethite application to the artificial substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on the concentration of arsenic (As) in different organs of Zea mays 32 d after planting. A different scale had to be selected for concentrations in the roots. Means for the different treatments were separated by Tukey test. For each plant organ, means followed by the same letter were not significantly different from each other.

View larger version (17K): [in this window] [in a new window]   Fig. 6. Effect of increasing amounts of goethite application to the artificial substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on shoot and root dry matter of Zea mays 32 d after planting. Means for the different treatments were separated by Tukey test. For each plant organ, means followed by the same letter were not significantly different from each other.

View larger version (33K): [in this window] [in a new window]   Fig. 7. Effect of increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on soil solution pH at increasing distance from the center of the root compartment with time. Horizontal hair lines at 16 mm distance from the center of the root compartment represent the location of the nylon net (root surface). Soil solution was sampled 10, 17, 24, and 31 d after planting (DAP).

View larger version (26K): [in this window] [in a new window]   Fig. 8. Effect of increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on soil solution iron (Fe) concentration at increasing distance from the center of the root compartment with time. Horizontal hair lines at 16 mm distance from the center of the root compartment represent the location of the nylon net (root surface). Soil solution was sampled 10, 17, 24, and 31 d after planting (DAP).

View larger version (25K): [in this window] [in a new window]   Fig. 9. Effect of increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on soil solution arsenite (AsIII) concentration at increasing distance from the center of the root compartment with time. Horizontal hair lines at 16 mm distance from the center of the root compartment represent the location of the nylon net (root surface). Soil solution was sampled 10, 17, 24, and 31 d after planting (DAP).

View larger version (14K): [in this window] [in a new window]   Fig. 10. Effect of increasing amounts of goethite application to the artificial quartz substrate (G-0 = 0 g kg–1; G-1 = 1 g kg–1; G-4 = 4 g kg–1) on the relationship between soil solution pH and phosphate (PV) concentration over all sampling times and suction cup positions.