Long-term effects of heavy applications of biosolids on organic matter and nutrient content of a coarse-textured forest soil

Abstract

Long-term changes in soil properties due to a single heavy application of municipal biosolids (municipal sewage sludge) on a coarse-textured glacial outwash soil were evaluated. Study sites, located at the University of Washington's Pack Experimental Forest, 100 km south of Seattle, were clearcut, cleared, fertilized with 500 Mg ha −1 of municipal biosolids and planted a variety of tree species in 1975. Soil samples were taken in 1990 from three biosolids-amended forest stands and adjacent unamended control sites by horizon to a depth of 185 cm. Biosolids-amended samples had higher C (139 vs. 67 mg g −1), N (12 vs. 3.4 mg g −1), P (14 vs. 2.2 mg g −1) and S (2.5 vs. 0.4 mg g −1) contents in 0–7 cm mineral soil and other surface soil horizons compared with adjacent unamended soil horizons, but showed no significant differences below 25 cm. Soil pH ranged from 0.4 to 1.0 units lower in biosolids-amended vs. unamended soil throughout the sampled soil horizon. Soil cation exchange capacity was higher in the surface soil horizons (30 vs. 18 mmol ckg −1 in 0–7 cm soil), but there were no significant differences below 50 cm. Biosolids-amended samples had higher total Ca (13 vs. 6.1 mg g −1 in 0–7 cm soil) and K (1.9 vs. 1.5 mg g −1 in 0–7 cm soil) throughout the sampled soil profile. Total Mg was relatively constant (2.0–3.0 mg g −1) throughout the sampled soil profile. Study results indicate that one of the primary objectives of the original biosolids application (increasing total nutrients in the rooting zone of the forest soil) extended at least 15 years from the application date.