Carbon and nutrient release from anaerobic digestate solids applied as a soil amendment

Abstract

Anaerobic digestion can produce renewable natural gas and is a viable alternative to conventional sources. When anaerobic digesters are coupled with agricultural systems, the resulting anaerobic digestate solids (ADS) after biogas production can be applied to fields as a fertilizer and an organic soil amendment. Therefore, ADS can potentially increase soil organic carbon (SOC) stock and improve soil fertility. To better understand the impacts of ADS on SOC accumulation and nutrient release, we conducted a 120-day laboratory incubation using four ADS rates (0, 2.5, 5, and 10 Mg C ha-1) in typical loamy and sandy soils of Iowa. We measured respired CO2-C, δ13CO2-C, dissolved reactive phosphorus (DRP), and extractable nitrogen (N). ADS-derived CO2 increased, but SOC-derived CO2 decreased as the ADS rate increased, indicating a negative priming effect (average of -78%). The C balance in the soil, defined as C inputs minus C respiration, significantly increased with ADS rates. Using reasonable bulk density and mixing depth assumptions, applying the medium ADS rate to soil would accumulate more SOC in the sandy than in the loamy soil (3 vs. 2.2 Mg C ha-1). Extractable N and DRP release rates were affected by ADS rates but in opposite directions. DRP increased while extractable N decreased with ADS additions. We conclude that ADS is a bioavailable source of C and nutrients for soil microbes that decreases short-term inorganic N, increases phosphorus availability, and leads to SOC accrual.