Temporal variability in trace metal solubility in a paddy soil not reflected in uptake by rice (Oryza sativa L.)

Abstract

Alternating flooding and drainage conditions have a strong influence on redox chemistry and the solubility of trace metals in paddy soils. However, current knowledge of how the effects of water management on trace metal solubility are linked to trace metal uptake by rice plants over time is still limited. Here, a field-contaminated paddy soil was subjected to two flooding and drainage cycles in a pot experiment with two rice plant cultivars, exhibiting either high or low Cd accumulation characteristics. Flooding led to a strong vertical gradient in the redox potential (Eh). The pH and Mn, Fe, and dissolved organic carbon concentrations increased with decreasing Eh and vice versa. During flooding, trace metal solubility decreased markedly, probably due to sulfide mineral precipitation. Despite its low solubility, the Cd content in rice grains exceeded the food quality standards for both cultivars. Trace metal contents in different rice plant tissues (roots, stem, and leaves) increased at a constant rate during the first flooding and drainage cycle but decreased after reaching a maximum during the second cycle. As such, the high temporal variability in trace metal solubility was not reflected in trace metal uptake by rice plants over time. This might be due to the presence of aerobic conditions and a consequent higher trace metal solubility near the root surface, even during flooding. Trace metal solubility in the rhizosphere should be considered when linking water management to trace metal uptake by rice over time.

Keywords: Bioavailability; Paddy soils; Redox potential; Trace metal contamination; Uptake; Water management.

Fig. 1

Experimental setup of the pots used to monitor the Eh and chemical composition of the soil solution in the pot experiment with the two rice plant cultivars

Fig. 2

Harvesting times of the rice plants during the major rice growth stages and alternating flooding and drainage periods in the pot experiment. The first drainage period of 8 days was after 58 days of flooding, and the second drainage period of 16 days was after 46 days of flooding

Fig. 3

Eh, pH, and Mn, Fe, S, and DOC concentrations in the soil solution for the pot experiment with rice plant cultivar A159 (+3 cm refers to 3 cm above the interface, 0 cm equals the interface between the soil surface and the water layer, and −5, −10 cm, −20 cm refer to 5, 10, and 20 cm below the interface, respectively). The numbers at the top x-axis of the two upper figures indicate the time at which the drainage and flooding periods started. In figure with Eh data, 1–4 refer to the following reactions: (1) denitrification of NO₃ ⁻ to N₂, (2) and (3) reductive dissolution of MnO₂ and Fe(OH)₃ to Mn²⁺ and Fe²⁺, respectively, and (4) reduction of SO₄ ²⁻ to S²⁻

Fig. 4

Total dissolved concentrations of Cu, Cd, Pb, Zn, and Ni in the soil solution for the pot experiment with rice plant cultivar A159 (+3 cm refers to 3 cm above the interface, 0 cm equals the interface between the soil surface and the water layer, and −5, −10 cm, −20 cm refer to 5, 10, and 20 cm below the interface, respectively). The numbers at the top x-axis of the two upper figures indicate the time at which the drainage and flooding periods started

Fig. 5

Biomass of the roots and the above-ground plant tissues including brown rice, husks, leaves, and stem and trace metal contents in these plant tissues for rice cultivar A159 during the major rice growth stages. Husks and brown rice were only harvested at the filling stage and final harvest. The numbers at the top of the figure indicate the time at which the drainage and flooding periods started

Fig. 6

Photographs taken from the rice plants harvested at the rice tillering stage (day 38) showing iron plaque formation on the root surface of rice plant cultivar A159

Fig. 7

Eh, pH, and Mn, Fe, S, and DOC concentrations in the soil solution for the pot experiment with rice plant cultivar A16 (+3 cm refers to 3 cm above the interface, 0 cm equals the interface between the soil surface and the water layer, and −5, −10 cm, −20 cm refer to 5, 10, and 20 cm below the interface, respectively). The numbers at the top x-axis of the two upper figures indicate the time at which the drainage and flooding periods started. In figure with Eh data, 1–4 refer to the following reactions: (1) denitrification of NO₃ ⁻ to N₂, (2) and (3) reductive dissolution of MnO₂ and Fe(OH)₃ to Mn²⁺ and Fe²⁺, respectively, and (4) reduction of SO₄ ²⁻ to S²⁻

Fig. 8

Total dissolved concentrations of Cu, Cd, Pb, Zn, and Ni in the soil solution for the pot experiment with rice plant cultivar A16 (+3 cm refers to 3 cm above the interface, 0 cm equals the interface between the soil surface and the water layer, and −5, −10 cm, −20 cm refer to 5, 10, and 20 cm below the interface, respectively). The numbers at the top x-axis of the two upper figures indicate the time at which the drainage and flooding periods started

Fig. 9

Biomass of the roots and the above-ground plant tissues including brown rice, husks, leaves, and stem and trace metal contents in these plant tissues for rice cultivar A16 during the major rice growth stages. Husks and brown rice were only harvested at the filling stage and final harvest. The numbers at the top of the figure indicate the time at which the drainage and flooding periods started