Soil redox-pH stability of arsenic species and its influence on arsenic uptake by rice

Abstract

Arsenic absorption by rice (Oryza sativa, L.) in relation to As chemical form present in soil solution was examined. Rice plants were grown in soil suspensions equilibrated under selected conditions of redox and pH, affecting arsenic solubility and speciation. A decrease in pH led to higher dissolved arsenic concentrations. When the soil redox potential dropped below 0 mV, most of the arsenic was present as As(Ill). Under more oxidizing conditions both As(Ill) and As(V) are present. Chemical speciation of As in the watersoluble fraction affected its phytoavailability. Most indigenous arsenic taken up by the plants remained in the root. Plant arsenic availability increased with increasing arsenic concentration in solution (lower soil pH) and with increasing amounts of soluble As(Ill) (lower soil redox). We also studied the uptake of monomethyl arsenic acid (MMAA), a widely used defoliant and herbicide, as affected by soil redox-pH condition. Amended MMAA was approximately two times more phytoavailable than the indigenous inorganic As forms and increased with decreasing pH and redox.

Effects of water management on arsenic and cadmium speciation and accumulation in an upland rice cultivar

“…Relatively aerobic conditions (intermittent and intermittent-aerobic treatments in the pot experiment) substantially reduced As accumulation in the shoots. Similar results have been obtained in other studies (Marin et al, 1993;Li et al, 2009b;Somenahally et al, 2011). However, Cd concentrations in straw, husk and brown rice decreased markedly in higher irrigation regimes (flooding and conventional) compared to aerobic and intermittent treatments under field conditions.…”

“…Similarly, Xu et al (2008) reported that during the period of active rice growth the As concentration in the soil solution was 4-13 times higher under flooded than that under aerobic conditions. It was also reported that in aerobic conditions the soil redox potential increased and arsenite was oxidized to arsenate (Marin et al, 1993). Arsenate adsorbs to the surfaces of iron oxyhydroxides and this decreases the release of As into the solution phase .…”

“…Thus, flooding during the growing season, especially during later stages of plant growth, can effectively reduce Cd concentrations in rice grains (Arao et al, 2009;Hu et al, 2013a). In contrast, anaerobic conditions in paddy soils usually lead to the reduction of arsenate (As(V)) to arsenite (As(III)) which enhances the bioavailability of As to the rice plants (Masscheleyn et al, 1991;Marin et al, 1993;Takahashi et al, 2004). Thus, growing rice aerobically results in a marked decline in As accumulation in the rice Li et al, 2009b;Hua et al, 2011;Somenahally et al, 2011;Spanu et al, 2012).…”

Effect of phosphorus on arsenic uptake and metabolism in rice cultivars differing in phosphorus use efficiency and response

“…On the other hand under P starvation this cultivar had over 35% higher As concentration in shoot tissue as compared to IRGA 423 and 424 (Figure 4). Arsenite (As(III)) is the dominant As species in reducing environments such as flooded paddy soils (Marin et al 1993, Takahashi et al 2004, Xu et al 2008. Thermodynamically, reduction of arsenate to arsenite can occur quite readily at intermediate redox potentials (Inskeep et al 2002).…”

Mitigation of cadmium and arsenic in rice grain by applying different silicon fertilizers in contaminated fields

“…The soil pH substantially affects the bioavailability of As and Cd and their absorption by plants (Reddy and Patrick 1977;Masscheleyn et al 1991;Marin et al 1993;Naidu et al 1994;Chan et al 2008). The soil was slightly acidic with pH 6.0 (Table 3).…”