Broiler litter application rate and soil depth effects on arsenic and phosphorus adsorption to soil

The Ozark Highlands, particularly northwest Arkansas, is one of several regions in the United States with concentrated broiler (Gallus gallus domesticus) production. In Arkansas alone, over 1.2 billion broilers are raised annually, which also generates ∼1.7 million Mg of broiler waste (i.e., litter) every year. Broiler 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, particularly arsenic (As). Adsorption to soil particles is a primary means of As and phosphorus (P) retention in soil that limits As and P mobility and the potential threat to 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 of northwest Arkansas and the Ozark Highlands in general. 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-1] applied for four consecutive years, and soil depth (0- to 20- and 20- to 50-cm) on As and P soil adsorption characteristics in the presence or absence of a small concentration of P and As, respectively, in solution. In both As and P experiments, the suspension pH increased as the litter treatment increased for the high, low, and control litter treatments, and increased more in the 20- to 50-cm depth than to the 0- to 20-cm depth. Arsenic adsorption was greater to the soil of the control (0 Mg ha -1) than to the soil of the low (5.6 Mg ha-1) and high (11.2 Mg h-1) 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. Arsenic adsorption in the presence of added P was nearly two-fold than when only As was present. However, no conclusions could be drawn from the P only experiment due to the narrow equilibrium P concentrations achieved (∼0 to 5 mg P L-1. These results 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 to groundwater contamination from increased As leaching may also be less than once thought.