Nitrogen dynamics and carbon sequestration in soil following application of digestates from one- and two-step anaerobic digestion

• Published in: The Science of the total environment 2022-12, Vol.851, p.158177-158177, Article 158177
• Main Authors: Nyang'au, Jared Onyango, Møller, Henrik Bjarne, Sørensen, Peter
• Format: Article
• Language: English
• Subjects: anaerobic digestion; Biogas; biomass; carbon; Carbon mineralisation; Carbon model; carbon sequestration; environment; greenhouse gases; Hydraulic retention time; loamy sand soils; mineralization; nitrogen; nitrogen fertilizers; Nitrogen mineralisation; organic matter
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2022.158177

Anaerobic digestion (AD) is an important tool for reducing greenhouse gas emissions from agricultural production. A prolonged retention time by adding an extra anaerobic digestion step can be utilized to further degrade the digestates, contributing to increased nitrogen mineralisation and reducing decomposable organic matter. These modifications could influence the potential N fertiliser value of the digestate and soil carbon sequestration after field application. This study investigated the effects of prolonging retention time by implementing an additional anaerobic digestion step on carbon and nitrogen dynamics in the soil and soil carbon sequestration. Two digestates obtained from two biogas plants operating at contrasting hydraulic retention times, with and without an additional digestion step, were applied to a loamy sand soil. N mineralisation dynamics were measured during 80 days and C mineralisation during 212 days. After 80 days of incubation, the net inorganic N release from digestates obtained from a secondary AD step increased by 9–17 % (% of the N input) compared to corresponding digestates obtained from a primary AD step. A kinetic four-pool carbon model was used to fit C mineralisation data to estimate carbon sequestration in the soil. After 212 days of incubation, the net C mineralisation was highest in undigested solid biomass (68 %) and digestates obtained from the primary AD step (59–65 %). The model predicted that 26–54 % of C applied is sequestered in the soil in the long-term. The long-term soil C retention related to the C present before digestion was similar for one- and two-step AD at 12–16 %. We conclude that optimizing the anaerobic digestion configurations by including a secondary AD step could potentially replace more mineral N fertiliser due to an improved N fertiliser value of the resultant digestate without affecting carbon sequestration negatively. [Display omitted] •A secondary anaerobic digestion (AD) step increases net inorganic N release from digestates in soil.•Two-step AD decreases digestate carbon mineralisation in soil due to extended decomposition in the AD process.•The fertiliser value of digestates can be improved by prolonged digestion time without affecting the long-term soil C retention.•Effects of a second step AD on C and N dynamics are dependent on hydraulic retention time utilized in the primary AD step.