Effect of Land-applied Arsenicals on the Soils of Alabama

Abstract

Many agricultural practices apply arsenicals directly to land through agricultural practices. Over time, repeated land-application of arsenicals may result in accumulation of arsenic in the soil. This may be problematic especially when arsenicals are added with poultry manure, which contains phosphorus and organic matter that may increase arsenic solubility. In Alabama, little is known about the consequences of repeated application of arsenicals to the soil. The objectives of this study were to: 1) assess the extent of arsenic contamination with repeated poultry litter or arsenical herbicide application and 2) evaluate the impact of soil phosphorus content on arsenite, arsenate, roxarsone, and monosodium methane arsenate (MSMA) sorption and solubility. For objective 1, sites with more than 10 years of repeated application of poultry litter and MSMA were sampled to determine the accumulation and distribution of arsenic within the profile. Soil samples at four different depths were characterized for soil pH, Mehlich-I extractable phosphorus, non-crystalline iron oxide fraction, total carbon content, and total soil arsenic content. With a few exceptions, there was a homogenous distribution of soil pH and non-crystalline iron oxide fraction with no definite pattern at sampled locations across depths. Phosphorus content ranged from 2.94 to 183.4 mg kg-1. Total carbon content decreased from surface to subsoil. Soil arsenic content varied among sampled fields at location 1 and 2 and at certain depths at Sand Mountain Research and Extension Center (SMREC). At Tennessee Valley Research and Extension Center, soil arsenic concentrations were consistent and near expected background levels. Although differences in soil arsenic concentrations were detected among fields at some locations, contamination of soils as a result of long-term poultry litter or MSMA application was inconclusive. Some control fields that did not have a history of poultry litter or arsenical herbicide application, had elevated levels of arsenic. This may be due to cotton production many years ago. Arsenicals were historically used as defoliants, herbicides, and pesticides and may still cause elevated background readings. For objective 2, batch experiments were conducted on low and high phosphorus soils from an 80-year fertilization study at the SMREC in northern Alabama. Phosphorus loading had no influence on the sorption of arsenite, arsenate, roxarsone, or MSMA. Arsenite had greater sorption than arsenate. Roxarsone and MSMA did not sorb to soil colloids and resulted in slight desorption of native arsenic from soil. Thus, the potential for leaching and mobility of arsenic land-applied to Hartsells soils is much greater for arsenate, roxarone, and MSMA than arsenite.