Adding anaerobic digestate to commercial farm fields increases soil organic carbon

Integrating anaerobic digestion technology can help farms meet agronomic, environmental, and economic goals. The slurry by-product from anaerobic digestion – anaerobic digestate – can be applied to croplands as an organic amendment and fertilizer. However, digestate effects on soil organic carbon (SOC) and soil fertility are poorly understood, especially at the field scale. In this study, we analyzed data from a multi-field commercial farm in Iowa, USA, which integrates agricultural production with anaerobic digestion. The digestate produced in this system was applied to 14 crop fields over 5–12 years. To assess the digestate effects on SOC stocks and soil plant-available nutrients, we analyzed the digestate chemical composition, digestate rates, and soil test results of 421 samples taken at 0–15 cm soil depth. Most sampled points (86 %) increased SOC stock, with greater gains observed in soils with lower initial SOC levels. The average SOC accrual rate was 0.8 Mg ha −1 y −1 (confidence interval: 0.6–0.9 Mg ha −1 yr −1 ), and isotope analysis ( 13 C and 15 N) indicated that new soil organic matter is primarily derived from digestate. Assuming simplified SOC dynamics, the SOC formation efficiency from digestate was estimated at 18 % (higher than the estimate for raw manure). Anaerobic digestate also increased soil test phosphorus (STP) and potassium, with STP values doubling over eight years, exceeding crop requirements. Integrating anaerobic digestion on farms can help reverse soil degradation and enhance agricultural sustainability, although STP should be monitored to prevent potential adverse environmental impacts. • Anaerobic digestate increased soil organic carbon (SOC) in commercial fields. • Soils with lower initial SOC stocks showed greater increases in SOC stocks. • The average SOC accrual rate across all fields was 0.8 Mg ha −1 y −1 . • Adding digestate also increased soil test phosphorus (STP) and potassium. • Digestate increases SOC but STP should be monitored to prevent water quality issues.