Arsenic in wetland vegetation: Availability, phytotoxicity, uptake and effects on plant growth and nutrition

In wetland surface sediments of Louisiana, arsenic (As) concentrations are elevated because of a wide use of inorganic arsenicals as cotton desiccants and of organic arsenicals as herbicides in rice-producing areas. Beside this, As levels are even higher in the region of produced water discharge associated with petroleum hydrocarbon recovery operations. The uptake, potential bioavailability and phytotoxicity of As to an important wetland plant species, growing in the vicinity of produced water discharge sites, were studied. The effects caused by As chemical form and concentration on growth, tissue concentrations and distribution of As and nutrient elements were studied in Spartina alterniflora growing in hydroponic conditions. A 4 x 4 factorial experiment was conducted with treatments consisting of four As chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsonic acid, MMAA; and dimethylarsinic acid, DMAA] and four As concentrations (0, 0.2, 0.8 and 2.0 mg As l(-1)). Arsenic phytoavailability and phytotoxicity were primarily determined by the As chemical form present in the nutrient solution. MMAA was the most phytotoxic species to this marsh grass. Regardless of the chemical form, an As level in the nutrient solution of 0.2 mg l(-1) was safe or caused no toxic effects for this marsh grass (it did not reduce plant growth or interfere with plant nutrition). In fact, inorganic arsenicals significantly increased total dry biomass production at an application rate of 0.2 mg l(-1). Arsenic availability followed the trend DMAA << MMAA < As(V) < As(III). Root and shoot As concentrations significantly increased with increasing As application rates (all four species) to the rooting medium. Inorganic arsenicals and MMAA were mainly accumulated in roots, while DMAA was readily translocated to shoots. Arsenic chemical form and concentration significantly affected macro- and micro-nutrient concentrations in plant tissue. Plants treated with As(V) had an improved growth compared to control plants; this seemed to be associated to an increase in plant P concentrations. Organic arsenicals caused the highest Na root concentrations and simultaneously the lowest plant K levels (antagonism K-Na). A significant increase in leaf Ca concentrations was found when organic As species were applied; this could have been due to the protective action of this nutrient to metals and metalloids toxicity. Inorganic arsenicals significantly increased the concentrations of B (root), Cu (root) and Cu (shoot). The high phytotoxicity of the MMAA treatments could have been related to the significant reductions in the concentrations of several essential macronutrients P, K, Ca and Mg and micronutrients B, Cu and Fe. (C) 1998 Elsevier Science B.V. All rights reserved.