Quantifying and Modeling Preferential Flow and Nutrient Loss from Agricultural Soils Fertilized with Poultry Litter.

Abstract

Fertilizing agricultural fields with poultry litter to meet crop nutrient requirements and using cover crops to improve soil health are important agricultural management practices. Excessive loss of contaminants via surface runoff and subsurface flow pathways from fields fertilized with poultry litter can cause water quality impairment. Further, recent research has shown that cover crops, while beneficial for soil health, can increase macroporosity in soil layers, the size of the large pores, the interconnectivity, and connection probability of larger pores, which may significantly enhance the transport of contaminants via preferential flow pathways. Therefore, it is crucial to understand the dynamics of the transport of pollutants (e.g., phosphorus and trace metals (copper, zinc, and nickel)) in agricultural fields. Specific objectives of this study were to (1) investigate the effect of cover crops and poultry litter application on phosphorus, zinc, copper, and nickel losses via leaching and (2) model the degree of preferential flow occurring and transport of trace metals (copper and zinc) in undisturbed soil columns with surface-applied poultry litter in pastures. For objective 1, ten intact undisturbed soil cores (150 mm diameter and 500 mm length) collected from E.V. Smith Research Center, near Shorter, Alabama, were used for laboratory-based leaching experiments. For the leaching experiments, the treatments included cover crop and no-cover crop soil columns with and without (control) poultry litter application. The leachates collected during the rainfall simulation experiments were analyzed for total metals, dissolved metals, colloidal metals, and dissolved reactive phosphorus (DRP). The results showed preferential flow in the columns due to the presence of macropores. The total, colloidal, and dissolved phosphorus concentration was significantly higher in columns with cover crops fertilized with poultry litter than in columns with no-cover crops fertilized with poultry litter. Similar trends were observed in trace metal losses. The preferential flow through soil macropores increased the mobility and velocity of solute movement in cover cropping systems. For the second objective, a numerical model (HYDRUS-1D) was used to model the tracer and trace metal flow through the soil columns collected from the Sand Mountain region in northern Alabama. The inverse parameter optimization technique was used to obtain parameters that can shed light on the physical and chemical processes occurring in the flow media. The results showed that soil cores, though collected from the same pasture field, varied in their pore structures, which led to different degrees of preferential flow and solute transport in the vadose zone. The preferential flow occurring in the columns causes physical nonequilibrium (PNE). The mobile immobile model (MIM) in the HYDRUS-1D program successfully described the tracer transport and preferential flow occurring in the columns. Trace metal transport was accompanied by chemical nonequilibrium (CNE) due to the reactive nature of trace elements. A dual porosity model with PNE and CNE could model the preferential flow and kinetic desorption. PNE and CNE occurring simultaneously are difficult to distinguish as they affect each other in natural soils. However, numerical modeling allowed us to differentiate between the types of nonequilibrium and understand the factors affecting these nonequilibrium processes. Overall, understanding the mechanism of solute transport through the macropores is of utmost importance to develop appropriate management practices.