Comparison of Effects of Biosolids Versus Chemical Fertilizer Application On Nitrate Leaching in a Midwest Soil.

Land application of organic materials as a nutrient source for crop production needs to be managed to minimize nitrate (NO₃^-) leaching to groundwater.  To assess the impact of biosolids application on NO₃^- leaching, soil samples were taken in 1976 and 2008 from six depths (0-15, 15-30, 30-45, 45-60, 60-75 and 75-90 cm) at a long-term biosolids field experiment established on calcareous mined soil (Mesic Alfic Udarents) located in Fulton County, Illinois.  The experiment included application of four treatments annually from 1973 to 2008:  17, 34, and 68 Mg ha^-1 biosolids and 336 kg N ha^-1 chemical nitrogen (N) fertilizer.  The biosolids were applied as liquid biosolids from 1973 to 1984 and air-dried biosolids from 1985 to 2008.  Corn was grown annually throughout the study.  At four years of annual application of liquid biosolids, the NO₃^- concentrations in subsoil depths (15-90 cm) at the lowest rate of biosolids application (17 Mg ha^-1), which is equivalent to the agronomic rate, were similar to concentrations in the chemical fertilizer treatment.  Elevated subsoil NO₃^- concentrations above that in the fertilizer treatment in the samples collected in 1976 were observed only at the highest rate of biosolids application (68 Mg ha^-1).  At 36 years of annual application (12 years of liquid biosolids and 24 years of air-dried biosolids), the NO₃^- concentrations in the 45-90 cm depths were lower in all biosolids plots than in the chemical fertilizer treatment.  The NO₃^- concentration in the soil profile increased with depths from 15-30 cm to 75-90 cm after long-term use of chemical fertilizer, but this trend did not occur in the biosolids treatments.  The results indicate biosolids-amended soil appear to conserve the excess reactive N as organic N.  Our findings suggest that N leaching with long-term use of biosolids as N fertilizer is lower than with chemical fertilizer.