Evaluating solids, phosphorus, and nitrogen cycling and transport in vegetative treatment systems used for runoff control on beef feedlots

Runoff from open lot animal feeding operations has been recognized as a potential pollutant to receiving surface waters. This effluent is known to contain nutrients such as nitrogen and phosphorus, as well as other potential pollutants such as organic matter, solids, and pathogens. Increased environmental awareness has prompted the need for improved feedlot runoff control. As a result, open feedlots of all sizes are looking for cost effective alternatives to handle feedlot runoff. Vegetative treatment systems (VTSs) have been proposed as a potential option to control this runoff, enhance environmental security, and protect water quality. Although previous research has shown that vegetative treatment systems can be effective in plot-scale and limited field-scale studies, questions about their performance on commercial operations remain. In addition the sustainability and the mechanisms by which treatment is occurring are still uncertain. Answering questions about the mechanisms these systems use to provide treatment offers the possibility of improvements in future designs and will increase our ability to effectively operate and manage existing systems. Thus, the objectives of this research was to evaluate solids, phosphorus, and nitrogen transport and cycling within the vegetative treatment system to better understand that fate of these contaminants, and in doing so to improve the design and management of vegetative treatment systems. This dissertation will consist of work on each of these areas, solids, phosphorus, and nitrogen transport and cycling, as they, along with hydrology, are the keys to understanding vegetative treatment system performance and sustainability. The first section, on solids transport consists of three manuscripts. The first manuscript, Using total solids concentration to estimate nutrient content of feedlot runoff effluent from solids settling basins, vegetative infiltration basins, and vegetative treatment areas, relates nutrient content in feedlot runoff from solid settling basins, vegetative infiltration basins, and vegetative treatment areas to the solids content within the effluent. This analysis serves the purpose of demonstrating that managing and understanding the sedimentological connections within the treatment system provides a great deal of insight into transport of other parameters (particularly nitrogen, phosphorus, and organic matter). Specifically, this work demonstrates that if detailed models of sediment export from the feedlot and through the treatment system can be developed, then this information can be used in predicting the movement of other parameters of concern. The second manuscript, A review of settling characteristics of solids in runoff from beef feedlots reviews the sediment characteristics that are required to perform detailed modeling of solids transport within the treatment system. Specifically, the manuscript reviews the physical characteristics (particle size, density, and settling rates) of particles transported in runoff from beef feedlots, addressing how these properties differ between various feedlots with different surface conditions (concrete and earthen) and at different locations. The review focuses on the implications these settling properties have for designing successful sedimentation systems and in predicting the actual performance of settling basins. The third manuscript, Development of a runoff and sediment routing model for open lot beef feeding facilities describes the development of a hydraulic and sediment routing model designed to predict solids transport from feedlot surfaces. This model can be used for prioritizing feedlots that are in need of enhanced runoff control systems, evaluating the hydraulic and sediment loadings that a feedlot runoff control systems are required to handle, and for exploring how different feedlot sizes, layouts, and designs impact solids transport. The second section, on phosphorous fate and cycling in the vegetative treatment areas, consists of a series of three manuscripts that utilize different monitoring procedures and assays to assess mechanisms of phosphorus treatment and its fate within the vegetative treatment area. The first manuscript uses a phosphorus mass balance approach to project phosphorus accumulation in the soil and compares the projected increases to monitored trends in soil test phosphorus at six vegetation areas in Iowa. The manuscript provides a preliminary phosphorus balance at six vegetative treatment areas focusing on how phosphorus is partitioning between soil, water, and vegetation. The second manuscript builds on this work by utilizing a sequential fractionation procedure, the Hedley method, to better understand the accumulation patterns of phosphorus within the soil and thereby obtain the relative stability of the accumulated phosphorus. Results of the fractionation procedure were interpreted based on the concept that a maximum soil phosphorus retention capacity existed; however, none of the soils as of yet exhibited a phosphorus accumulation pattern indicative of saturation, although in many cases, specific pools, mostly organic phosphorus pools, did appear saturated. The third manuscript utilizes a phosphorus sorption experiment to evaluate how the soil\u27s phosphorus retention properties had been modified by five years of use as vegetative treatment areas. Specifically, the experiment evaluated how continued use of the vegetative treatment area modified the soil properties and the impact this had on the estimated phosphorus sink capacity of the soil. This experiment provides an evaluation of whether the life expectancy model developed previously by Baker et al. (2010) provides a useful estimation of vegetative treatment area phosphorus saturation life and explores what mechanisms may be allowing further phosphorus accumulation. Finally, the third section, on nitrogen transport and cycling, contains two manuscripts. The first manuscript, Vegetative treatment system impacts on groundwater quality, discusses groundwater concentrations up-gradient, within, and down-gradient of six vegetative treatment system on beef feedlots in Iowa. The manuscript provides statistical comparisons and trend analysis to evaluate impacts the system may be having. Nitrate leaching in the vegetative treatment system is also estimated. The second and final manuscript, The impact of vegetative treatment area use on soil biologically available carbon and nitrogen pools, reports the results of a long-term carbon and nitrogen fractionation procedure to evaluate if accumulation of labile carbon and nitrogen is occurring and if this organic matter is nitrogen enriched. A final conclusions manuscript, Vegetative treatment systems: design, management, and siting recommendations provides recommendations on what is required to construct successful vegetative treatment systems and which areas require future research so that designs can be refined and ensure appropriate nutrient cycling and retention