Effect of poultry litter application and preferential flow on metal loss in pastures

Large amounts of poultry litter, which is a mixture of manure and bedding material, are produced by the expanding poultry industry in the southeastern United States and are applied to pasture lands as an organic fertilizer. In addition to essential plant nutrients-nitrogen, phosphorus, and potassium (N-P-K)-poultry litter contains other nutrients and metals. However, information on the leaching potential of these metals from no-till pasture soils is scarce. This study addresses this critical gap by quantifying the subsurface loss of several metals (i.e., aluminum [Al], boron [B], calcium [Ca], magnesium [Mg], manganese [Mn], sodium [Na], and zinc [Zn]) following poultry litter application to tall fescue pastures, and evaluating the role of preferential flow paths in enhancing their mobility through the soil. A rainfall simulation was conducted on 12 undisturbed soil columns (150-mm diameter and 500-mm length) collected from a field at the Sand Mountain Research and Extension Center in Alabama, USA. Poultry litter was surface broadcasted at the application rate of 0 (control) and 5 Mg ha^-1 (treatment). Leachate metal concentrations were measured and bromide breakthrough curves (BTCs) were analyzed to assess the degree of preferential flow. A sequential extraction procedure for soil and poultry litter samples was used for identifying metals in the following seven categorized fractions: soluble, exchangeable, bound to carbonates, bound to amorphous oxides, bound to crystalline oxides, bound to organic matter, and residual. Shapes of the BTCs and other solute transport parameters, such as early breakthrough and immobile pore-water fraction (ranging from 0.31 to 0.80), provided direct evidence of preferential flow in the studied columns. The flow-weighted mean concentrations of Al, B, Na, and Zn in the leachate were significantly higher in the treatment columns than those in the control columns (e.g., Al: 0.82-3.33 mg L^-1, Zn: 0.03-0.16 mg L^-1, Na: 4-7.8 mg L^-1). However, poultry litter had no significant effect on metal leaching, except for Na. Eluted total metal loadings in the treatment columns varied in the following order: Ca > Mg > Na > Al > Zn > Mn > B. Although the leaching losses of certain metals did not significantly increase with poultry litter application in this study, their accumulation in the soil raises concerns. Despite not posing an immediate threat, the potential for soil contamination and the subsequent accelerated subsurface transport of these metals due to continued loading warrants further investigation. These findings have important implications for the safe and sustainable management of poultry litter in pasture soils.