The Age Effect of Dairy Feedlots on Manure Nutrient Seepage in Loam Soils

This project investigated the age effect of dairy feedlots, built on loam soils, on the nutrient seepage process. Soil samples from four dairy feedlots, 20, 40, 60, and 100 yr old, were taken both inside and outside the sites in about 30 cm depth intervals to 153 cm. It was found that for feedlots less than 20 yr old, only the topsoil layer (30 cm) had increased nitrate nitrogen concentration. For feedlots older than 20 yr, the seepage of nitrate nitrogen went deeper into the soil. The potential for pollution of groundwater by both ammonium and nitrate nitrogen may be reduced by moving animals to a new feedlot site every 40 yr if the water table is 61 cm below the ground surface. In addition, since the seepage rates for these two chemical species accelerate in the sampling depths from 92 to 153 cm as the feedlot age approaches 60 yr, continuing to use feedlots older than 60 yr is not recommended even if the water table is 153 cm below the ground surface. A linear relationship was observed between nitrate and ammonium concentrations in soil (coefficient of determination image). The leaching of phosphorus into loam soils was found to be a relatively slow process because only the topsoil layer (30 cm) had the raised phosphorus concentration after 60-yr operation, and therefore, is considered unlikely to contribute to groundwater pollution for feedlots less than 60 yr old.