Phosphorus Retention, Accumulation, and Movement in Six Feedlot Runoff Vegetative Treatment Areas

Abstract

Increased environmental awareness has prompted the need for improved feedlot runoff control. Vegetative treatment systems (VTSs) provide a cost effective option that may enhance environmental security. Vegetative treatment systems are typically designed on the basis of hydraulic performance, which may result in over-application of nutrients, especially phosphorus. This study assessed the retention, accumulation, and movement of phosphorus in vegetative treatment areas used for runoff control on six Iowa feedlots over a four year period. Phosphorus loadings and retention were calculated based on measured settled feedlot effluent, or vegetative infiltration basin, and vegetative treatment area runoff volumes and phosphorus concentrations. Results indicated that between 61 and 89% of all applied phosphorus was retained within the treatment area, resulting in phosphorus loadings of 124 to 358 kg P/ha-yr. Measurements of harvested vegetation phosphorus concentration and yield indicated that between 13 and 61 kg P/ha-yr were removed with vegetation harvest. However, this only accounted for 6 to 13% of all applied phosphorus, which suggests that these systems have potential for rapid phosphorus accumulation in surface soil, which could potentially lead to reduced treatment and loss of soluble phosphorus. Projected soil phosphorus accumulation was compared to annual measurements of soil Melich-3 phosphorus concentrations increases. Both approaches found similar increases in soil phosphorus levels, indicating that the majority of the phosphorus retained in vegetative treatment areas was due to interaction and retention in the surface soil. Deep soil sampling (0 to 122 cm) was utilized to evaluate vertical phosphorus movement of phosphorus through the soil profile. Sampling indicated that most accumulation was in the surface soil, but that signs of vertical transport and leaching were occurring after four years of operation especially near the VTA inlet.