Broiler-Litter Application History and Soil-Depth Effects on Arsenic Sorption

The Ozark Highlands, particularly northwest Arkansas, is one of several regions in the United States with concentrated broiler (Gallus gallus domesticus) production. Broiler litter contains N and P; thus, litter is typically land applied to surrounding pastures as a means of recycling the litter as an organic fertilizer. However, broiler litter also contains numerous heavy metals, namely arsenic (As). Sorption reactions are a primary means of As and P retention in soil that limits As and P mobility and the potential threat of surface and groundwater contamination. However, As also competes with P for adsorption sites under some soil conditions. Therefore, it is essential to understand how broiler litter-derived As behaves in a high soil-test P environment, which characterizes many of the pastures in the Ozark Highlands. The objective of this study was to evaluate the effects of broiler-litter application rate (0 [control], 5.6 [low], and 11.2 [high] Mg litter ha) applied for 4 consecutive years and soil depth (0-20 and 20-50 cm) on As sorption characteristics in the presence or absence of a small concentration of P in solution. Arsenic sorption was greater to the soil of the unamended control than to the soil of the low- and high-litter treatments and was also greater in the 20- to 50-cm depth than in the 0- to 20-cm depth whether additional P was present or not. However, As sorption in the presence of added P was nearly 2-fold that when only As was present. Results demonstrate that a single isotherm does not accurately depict As sorption to soil with a variable history of broiler-litter applications. Results also suggest that there may be less of a potential to increase As mobility in the soil when broiler litter is land applied containing both As and P. Thus, the potential threat of groundwater contamination from increased As leaching may also be less than once thought.