The effect of long-term annual application of biosolids on soil properties, phosphorus, and metals

The USEPA Part 503 rule did not directly specify the amount of sludge-borne P that can be applied but allowed application rates based on the recommended N requirement of a crop. Monitoring of long-term application of biosolids is important to protect both soil and water quality. Biosolids and ammonium nitrate were annually applied for 13 yr to winter wheat at six plant available N (PAN) rates: 0, 45, 90, 180, 269, 536 kg N ha(-1) yr(-1). Biosolids application did not increase soil pH but increased levels of total C, total N, NH4-N, NO3-N, and electrical conductivity (EC). Soil concentrations of Cd, Cu, Pb, Mo, and Zn were greater in biosolid-treated plots but were <24% of their respective cumulative pollutant loading rates. Application of biosolids increased plant micronutrient B, Cu, Fe, and Zn concentrations. The addition of biosolids to the Norge soil (fine-silty, mixed, active, thermic Udic Paleustoll) increased Mehlich 3 plant available P (M3P) and water soluble P (WSP) and these P levels were well correlated with biosolids application rate. Application of biosolids at rates of >= 2 times the recommended agronomic rate of 90 kg PAN ha(-1) resulted in M3P concentrations that exceeded an environmental threshold of 200 mg kg(-1) established by the USDA-NRCS for the land application of manures in Oklahoma non-nutrient limited watersheds. Significant relationships were found between the ammonium oxalate P saturation index (PSIox) and M3P as well as between PSIox and WSP. It appears the repeated longterm application of biosolids above the N agronomic rate should be avoided and application should be based on other criteria such as an agronomic P threshold, an environmental P threshold, or a P site index.