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

Summary

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%. Weconclude 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.