Long-Term Effects of Heavy Applications of Biosolids on Organic Matter and Nutrient Content of a Coarse-Textured Forest Soil

The long-term soil-chemistry effects were examined in a coarse-textured soil amended with biosolids and compared to an unamended soil. The biosolids were derived from anaerobically digested municipal sewage. The experimental sites were planted with Lombardy poplar, Douglas-fir, or ponderosa pine, and monitored over a 16-yr period. On the amended plots, there were significantly more highly humified material and fewer fine roots. Amendment resulted in significant increases in soil organic matter and nutrient concentrations. Base cation analysis revealed that cation leaching had occurred in the amended soil, especially for calcium, magnesium, and potassium, due presumably to the nitrification of organic nitrogen and ammonium added with the biosolids. The amended soil was more acidic than the unamended soil, which was attributed to high rates of nitrification.