Potential carbon and nitrogen mineralization in soils from a perennial forage production system amended with class B biosolids

The effects of biosolids application rate and history on soil potential C and N mineralization were measured over a 112-day laboratory incubation. Soils were collected from a large-scale biosolids recycling operation that surface-applies anaerobically digested Class B biosolids for commercial forage production. Five treatments were evaluated: unamended control; 22 Mg dry biosolids ha−1 y−1 applied for 25 years; 22, 45, and 67 Mg ha−1 y−1 applied for 8 years. Biosolids additions enhanced total soil organic C by 32–92% and total N by 30–157% compared to unamended soils. Total N increased with application rate and was dominated by nitrate-N. Potential C mineralization (cumulative CO2 produced) was 11–62% greater in amended soils compared to controls and highest at 67 Mg ha−1 y−1. Net N mineralization and immobilization were highest early in the incubation for 45 and 67 Mg ha−1 y−1 treatments. No significant differences in potential C and N mineralization between controls and soils amended at the lowest rate for 8 or 25 years suggests that biosolids applications at 22 Mg ha−1 y−1 are sustainable over the long-term. Higher potential N mineralization rates and soil nitrate concentrations under higher application rates may increase the risk of off-site nutrient transport and requires further evaluation.