Surface and Subsurface Transport of Phosphorus from Surface and Subsurface-applied Poultry Litter
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A plot scale experiment was conducted in a pasture at the Sand Mountain region of north Alabama, USA. Nine plots of 1.52 m by 3.05 m were constructed with sheet metal borders to isolate runoff. The soil was a Hartsells fine sandy loam (fine-loamy, siliceous, subactive, thermic Typic Hapludults) and the slope of the plots was 4%. A metal trough attached to 10 cm horizontal approach sheet metal was installed at the downslope end of the plot to collect surface runoff and convey it to a collection point at the corner of the plot. To collect leachate a pair of pan and wick lysimeter was installed 50 cm beneath the soil surface in each plot. There were three replications of each surface-applied, subsurface-banded litter plots, and three control (no litter) plots. For plots with surface application of litter, broiler litter was broadcast manually over the entire plot as uniformly as possible. For subsurface application of litter, subsurface band a applicator was used. Rainfall simulation was conducted at an intensity of 70 mm h-1 based on the 1 h, 10 year return period of that area. Runoff samples were collected every 5 minutes for 1 h after the start of runoff. Leachate samples were collected at the end of the rainfall event. Results showed that irrespective of treatment only 10% of the rainfall contributed to surface runoff. The method of litter application plays an important role in the concentration and loading of nutrients in surface runoff and leachate. Significantly (α = 0.05) greater concentration and loading of the nutrients (TP, PO4-P, NH4-N, and NO3-N) in runoff were observed from surface-applied litter plots than subsurface-banded. There was more than 80% reduction in concentration of TP and PO4-P in surface runoff when broiler litter was subsurface-banded in comparison with surface-applied litter plots. NH4-N and NO3-N concentrations were reduced by about 80 and 74%, respectively, in surface runoff from subsurface-banded litter plots in comparison with surface-applied plots. Similar trends in concentration were seen in the loading of nutrients in surface runoff when broiler litter was subsurface-banded. In leachate, concentration of TP and PO4-P reduced was by 37 and 95%, respectively, when broiler litter was subsurface-banded. The NO3-N concentration was reduced by 43% when broiler litter was subsurface-banded. Similar trends in concentration of nutrients in leachate were seen in the loading of nutrients in leachate. Subsurface application of litter reduced the concentration of TP and PO4-P to control plot levels in surface runoff and leachate. The concentration of TP and PO4-P was less in leachate in comparison with their corresponding concentration in surface runoff. However, since more than 90% of the water infiltrated, the loading of TP and PO4-P was greater in leachate in comparison with loading in surface runoff, irrespective of the treatment. PO4-P and TP loading was about 16 and 43%, respectively, more in leachate than surface runoff. This suggests that loss of phosphorus via subsurface flow is important in this region. Overall results of this study suggest that subsurface transport of P is as important as surface transport of P. New BMPs need to be developed which can reduce loss of nutrients via subsurface flow to a nearby waterbodies.