Greenhouse gas emissions from a sandy loam soil amended with digestate-derived biobased fertilisers – A microcosm study

Summary

Nutrient recovery from anaerobic digestion systems provides several side streams that are useful as biobased fertilisers (BBFs). A microcosm approach was employed to assess the short-term greenhouse gas emissions from a sandy-loam soil enriched with 18 BBFs in comparison with mineral fertilisers (urea and calcium ammonium nitrate). In total, 20 different fertilisers were homogeneously incorporated into an arable sandy loam soil at a rate of 170 kg nitrogen (N) ha⁻¹ and incubated at 80 % water-filled pore space. Over 18 days, the fluxes of nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) released in the headspace of the microcosms were measured using a Gasera One Multi-gas analyser. Cumulative N₂O emissions from the BBF treatments were either comparable or lower (0.04–0.09 %N applied) than the mineral fertilisers (0.10–0.14 %N applied). Nitrification of the initial ammonium-N present in the BBFs was likely the dominant biological process driving N₂O production. The application of digestate and evaporator concentrates led to an increase in CO₂ emissions (8–51 % of applied carbon (C)), mostly in the first days of the incubation. Meanwhile, the solid fraction of digestate exhibited slow mineralisation patterns (3–7 % of applied C). The variability in CO₂ respiration was strongly influenced by the availability of labile C. Fertilisation had no effect on soil-borne CH₄ emissions. Estimation of global warming potential, with respect to added N, suggests that BBFs obtained from the post-digestion treatment of digestate have a lower environmental impact compared to the unprocessed digestate, due to lower N₂O emissions.