The influence of redox potential and pH on arsenic speciation and solubility was studied in a contaminated soil. Alterations in the oxidation state of arsenic, as influenced by redox potential and pH, greatly affected its solubility in soil. At higher soil redox levels (500-200 mV), arsenic solubility was low and the major part (65-98%) of the arsenic in solution was present as As(V). An alkaline pH, or the reduction of As(V) to As(III), released substantial proportions of arsenic into solution. Under moderately reduced soil conditions (0-100 mV), arsenic solubility was controlled by the dissolution of iron oxyhydroxides. Arsenic was coprecipitated [as As(V)] with iron oxyhydroxides and released upon their solubilization. Upon reduction to -200 mV, the soluble arsenic content increased 13-fold as compared to 500 mV. The observed slow kinetics of the As(V)-As(III) transformation and the high concentrations of Mn present indicate that, under reduced soil conditions, arsenic solubility could be controlled by a Mn3(As04)2 phase.
Oxygen transport through the air spaces (aerenchyma tissue) of the stem and roots of aquatic macrophytes into the root zone supports nitrification of NH,+, with the NOJ-formed diffusing into the adjacent anaerobic zone where it undergoes #denitrification. To test this hypothesis, we conducted a growth chamber study to determine the transformation of applied 15NH,+-N to r5N, in the root zone of three aquatic macrophytes: rice (Oryza sativa L.), pickerel weed (Pontecferia cordata L.), and soft rush (Juncus e&us L.). Detection of gaseous 15N, in the air above the floodwater of the soil column with aquatic plants provided direct evidence of nitrification-dcnitrification in the root zone, while such losses were not measurable for soil columns without plants. Air spaces in aquatic plants can also function as conduits for denitrified gases from anaerobic sediments to the atmosphere. Maximal 15N, flux due to this process was 102, 113, and 122 mg N m-2 d-l for soft rush, rice, and pickerel weed. This N loss mechanism has important agronomic and ecological consequences.
Anaerobic soils released more phosphate to soil solutions low in soluble phosphate and sorbed more phosphate from soil solutions high in soluble phosphate than did aerobic soils. The difference in behavior of phosphate under aerobic and anaerobic conditions is attributed to the change brought about in ferric oxyhydroxide by soil reduction. The probably greater surface area of the gel-like reduced ferrous compounds in an anaerobic soil results in more soil phosphate being solubilized where solution phosphate is low and more solution phosphate being sorbed where solution phosphate is high.
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