The Effect of Long‐Term Annual Application of Biosolids on Soil Properties, Phosphorus, and Metals

Abstract

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 long‐term 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.