Preferential flow of phosphorus and nitrogen under steady-state saturated conditions

Repeated broiler litter application on agricultural lands can cause nutrient enrichment of subsurface effluent, especially with the existence of preferential flow through soil macropores. Previous studies quantifying soil macropores have not attempted to establish a connection of soil macropore characteristics with the subsurface nutrient (nitrogen [N] and phosphorus [P]) losses, across different topographical locations in the field. This study investigated the effect of broiler litter application and preferential flow on subsurface nutrient transport (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 computed tomography image analysis and solute transport modeling. Treatments included surface broadcast broiler litter (5 Mg ha-1, on dry basis) and unamended control. Leachates were analyzed for dissolved reactive P (DRP), total P (TP), and nitrate + nitrite-N (NO3- + NO2--N). The bromide breakthrough curves provided evidence of preferential flow in all columns. Litter application significantly increased leachate P concentrations, and average TP and DRP concentrations were significantly higher in the leachate from upslope columns compared to those at downslope location. The NO3--N concentrations in leachate exceeded the US EPA drinking water standard of 10 mg L-1 in all the treatment columns. The highest flow-weighted mean concentrations of TP and DRP, at 2.7 and 2.5 mg L-1, respectively, were recorded in the upslope columns. Soil physicochemical properties and nutrient leaching losses varied substantially across topographical positions, indicating a need for variable litter application rates to reduce P build-up and subsequent leaching in vulnerable locations within the field. The relevance of the effect of topographic position on nutrient leaching found in this study should be further tested by investigating a wider range of slopes and soil types in pastures. Phosphorus and nitrogen concentrations in the leachate varied as a function of slope position. Breakthrough curves and X-ray computed tomography (CT) analysis provided evidence of preferential flow. Solute transport was well-fitted by the two-region physical nonequilibrium model. Broiler litter application significantly increased phosphorus leaching under saturated conditions.