Date of Award

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

William H. Patrick, Jr

Abstract

Studies dealing with the availability and speciation of arsenic (As) as affected by soil redox potential and pH were initiated because of the lack of information on As chemistry in flooded soils. The chemistry of native and applied As was studied in a Crowley silt loam soil (Typic Albaqualf). Arsenic uptake and its toxic effect on two rice cultivars as affected by As chemical form and concentration were also studied. Soil redox potential and pH were shown to affect speciation and solubility of both native and applied As. Upon soil reduction, indigenous-As solubility increased, and arsenite (As(III)) comprised most of the soluble As. At the lowest redox potential ($-$200 mV) 7.3, 2.2 and 1.4% of soil As became soluble at pH's 5.5, 6.5 and 7.5 respectively. Under oxidized conditions, As solubility was lower, and arsenate (As(V)) constituted most of the soluble As. When 4 mg kg$\sp{-1}$ monomethylarsenic acid (MMAA) was added, a similar trend occurred. Solubility of applied MMAA increased when the soil was reduced. In the two experiments, the greater availability of As under reduced conditions led to a greater As uptake by rice plants. Studies with plants grown in nutrient solution showed As chemical form to be the most important factor determining As availability and toxicity. Arsenic phytoavailability followed the trend As(III) $>$ MMAA $>$ As(V) $>$ DMAA (dimethylarsenic acid), while As toxicity followed the trend MMAA $>$ As(III) $>$ As(V) $\sim$ DMAA. Arsenic taken up as MMAA, As(III) or As(V) was stored in the root, but As taken up as DMAA was readily translocated to the shoot. Phosphorus uptake decreased with increasing As application. Zinc tissue concentration and uptake was decreased by all chemical forms of As. In both the soil and the nutrient solution experiments, the uptake of As as MMAA interfered with the translocation of Zn and, to a lesser degree, Cu. Root applied DMAA at a concentration of 1.6 mg As L$\sp{-1}$ inhibited photosynthetic activity, photosynthetic capacity and plant growth, leading eventually to death. Photosynthetic activity was inversely related to tissue As concentration.

Pages

119

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