|dc.description.abstract||A significant amount of poultry litter produced in the southeastern United States is applied to pasture lands. Poultry litter application on agricultural lands can cause nutrient enrichment of subsurface effluent, particularly when preferential flow exists. In addition to surface runoff, recent research efforts have shown that phosphorus (P) loss can occur through leaching. Preferential flow has been increasingly recognized as an important pathway for water and nutrient transport, and macropore flow has been considered an important mode of preferential flow in many agricultural systems. Therefore, improving our knowledge of soil P transport dynamics is critical to management strategies that reduce P loadings to water bodies. Objective 1 of this study investigated the effect of broiler litter application and preferential flow on subsurface nutrient transport (nitrogen (N) and P) at different topographical positions (upslope, midslope, and downslope) in a no-till pasture field located in Alabama, USA. Twelve intact soil columns (150 mm ID and 500 mm length) were used, and the nutrient leaching measurements from laboratory experiments were linked to soil macropore characteristics quantified using X-ray CT image analysis and solute transport modeling. Treatments included surface broadcast broiler litter and unamended control. Leachates were analyzed for dissolved reactive P (DRP), total P (TP), and nitrite + nitrate-N (NO3- + NO2- - N). Transport parameters were estimated through inverse modeling of the two-region model using the CXTFIT program. The results showed that preferential flow existed in all columns. Litter application significantly increased leachate P concentrations, with the highest average TP and DRP concentrations being 2.7 and 2.5 mg L-1, respectively. The NO3--N concentrations in leachate exceeded the U.S. EPA drinking water standard of 10 mg L-1 in all the treatment columns. Soil physicochemical properties and nutrient leaching losses varied substantially across topographical positions, indicating a need for variable litter application rates to reduce nutrient inputs in nutrient-enriched locations within the field.
Apart from essential plant nutrients (N-P-K), poultry litter also contains other nutrients and trace or heavy metals, yet little is known about the potential of these metals to leach from no-till pasture soils. Objective 2 of this study aimed to quantify the subsurface loss of several metals (i.e., Al, B, Ca, Mg, Mn, Na, and Zn) after poultry litter application from a tall fescue pasture and determine if preferential flow paths enhance the mobility of these metals through the soil. A rainfall simulation study was conducted on six undisturbed soil columns (150 mm ID and 500 mm length) collected from a field at the Sand Mountain Research and Extension Center in Alabama. Poultry litter was applied at 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 was used for recognizing 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. The shape of the BTCs and other solute transport parameters, such as early breakthrough and immobile pore-water fraction, provided direct evidence of preferential flow in all the columns. Flow-weighted mean concentrations of B, Na, and Zn in the leachate were significantly higher for treatment columns compared to control columns. However, poultry litter had no significant effect on metal leaching, with the exception of Na. Eluted total metal loadings in the treatment columns varied in the following sequence: Ca > Mg > Na > Zn > Mn > Al > B. The BTCs of metals also indicated the presence of preferential flow. Although the leaching losses of some of the metals in this study did not increase substantially with poultry litter application, the retention of metals in soil suggests that the PL application does not pose an immediate threat, however, their potential to contaminate soil due to continued loading requires further investigation. These findings have important implications for the safe and sustainable management of poultry litter application on pasture soils.
Phosphorus can be transported in different forms, including dissolved, particulate, and colloidal forms. However, many water quality studies in agro-environmental sciences focus on analyzing DRP and/or TP, often neglecting the P associated with colloidal particles. However, colloidal phosphorus (Pcoll) constitutes an important mobile component of TP in subsurface drainage waters. Colloids are suspensions of finely dispersed particles ranging in size from 1 nm to 1 μm, and because of their large surface area relative to their mass, colloids exhibit a high sorption capacity for P and have been associated with the long-distance transport of P in both aquatic and terrestrial environments. Objective 3 of this study evaluated the vertical transport of different forms of P (dissolved, particulate, and colloidal) in undisturbed soil columns receiving three types of animal manure: solid poultry litter, swine lagoon effluent, and liquid dairy manure. Swine lagoon effluent resulted in greater Pcoll leaching than liquid dairy manure and poultry litter. A large portion of the TP in the leachate from columns with poultry litter and dairy manure application was composed of total reactive P. In contrast, for columns amended with swine lagoon effluent, most of the P in the leachate was comprised of total unreactive P, and P transport was primarily in the particulate form.
Preferential flow can be described with two-region or multi-region approaches to explain the physical and chemical nonequilibrium of water flow and solute transport. However, a major challenge in dual or multi-domain models is the difficulty in measuring or estimating a large number of parameters. Therefore, modeling approaches have proved useful in simulating conditions that are not feasible to replicate in field experiments due to economic or technical constraints. Utilizing inverse techniques to estimate parameters controlling nonequilibrium flow and solute transport in the soil profile can provide valuable insights and understanding of solute transport under preferential flow conditions, enabling accurate predictions and informed agricultural and environmental management decision-making. Therefore, objective 4 of this study was to utilize HYDRUS-1D as a tool to develop an improved understanding of the P transport processes and evaluate the applicability of dual-porosity with two-site sorption in mobile region physical and chemical nonequilibrium model in simulating P transport using six undisturbed soil columns. The results indicated that the dual porosity model fitted the P BTCs well and explained the P transport and reaction processes in the soil profile. Overall, the results showed that most of the sorption sites (~63%) were in contact with immobile water, and all of the sorption in the mobile zone was kinetic, which led to the rapid transport of P from the soil to the leachate. Two scenarios based on increased poultry litter application rate and initial soil P levels were developed and their effects on P leachate concentrations were compared. The simulations indicated that the elevated litter application has a much more substantial impact on P transport compared to soil P level, emphasizing the importance of considering litter management practices as a key factor in controlling P leaching and its potential environmental consequences in soils with the existence of preferential flow conditions.||en_US