Environmental Impact of Exceptional Quality Biosolids Use in Urban Agriculture

Exceptional quality (EQ) biosolids may be used to rehabilitate anthropogenic soils for urban agriculture, but research on the environmental ramifications of applying biosolids at rates to improve such degraded urban soils is scarce. The objective of our study was to compare the impacts of EQ biosolids and inorganic fertilizer applied for rehabilitating anthropogenic soils on potential nitrogen (N) and phosphorus (P) water quality impairment risk. Three EQ biosolids (composted, air‐dried, and blended with woody mulch) were applied at agronomic N rates twice per year, and once per year at a disturbed land reclamation rate (5× agronomic N rate) for vegetable production to an anthropogenic, low‐fertility subsoil. A heat‐dried EQ biosolids and inorganic fertilizer control were applied at agronomic N rates only. Leachate P was below detection, and soil water‐extractable P concentrations were low. After 2 yr, reclamation rates resulted in soil P saturation ratios greater than the recommended 30% but lower than the 65% limit. The high Fe in biosolids, low soil initial P, and high pH, exchangeable Ca, and clay content, contributed to lower soluble P. Although nitrate N (NO3–N) posed the greatest nutrient leaching risk, lysimeter‐collected mass loading of NO3–N from reclamation rates was not greater than for the inorganic fertilizer. Relatively low NO3–N losses with reclamation rates could be due to denitrification or physical protection of organic N in the high‐clay soil. Our results indicate that reclamation rates of our EQ biosolids to anthropogenic, low‐fertility, clayey subsoils have low potential for N and P water quality impairment. Core Ideas Risk of phosphorus (P) loss was low even after high P biosolids applications. Cumulative nitrate loads were similar between reclamation rates and inorganic fertilizer. Dissolved organic carbon concentrations increased after biosolids reclamation rates. Large amounts of soil nitrogen accumulated 2 yr after treatment additions.