Anaerobic digestate is a nutrient rich slurry by-product derived from biogas production, often used as a fertiliser due to its high nitrogen content. However, nitrogen losses from its application can lead to environmental pollution. In a laboratory experiment, the addition of high organic carbon materials to digestate-amended soil as a potential means to stimulate microbial immobilisation of digestate supplied nitrogen was investigated. Soil was incubated in pots for 5 months with digestate (equivalent to 250 kgN ha⁻¹). The impact of adding carbon into the digestate (equivalent to 540 kgC ha⁻¹) as either glycerol, straw, woodchip, or biochar on soil microbial and chemical parameters was quantified. Glycerol amended soils had significantly higher microbial biomass compared to digestate alone during the first month and at 30 days after application had a 4x higher on average microbial N. The digestate + straw treatment resulted in a 2.5x significantly greater nitrogen immobilisation compared to digestate alone after 3 months of incubation. The digestate + woodchip had a 2× higher mean microbial N after 5 months, whilst the biochar amendment did not stimulate significant nitrogen immobilisation at any time. These results suggest that mixing a labile to moderately labile organic carbon amendment, such as straw, with digestate has the greatest potential to reduce nitrogen losses following digestate application through microbial immobilisation.

Anaerobic digestate, a nutrient rich by-product of the biogas industry, is frequently applied to agricultural land as a fertiliser. However, nitrogen losses from its application negatively impact air and water quality. Therefore, methods are needed to reduce these losses. The aim of this study was to test the efficacy of applying digestate with glycerol, an organic carbon rich by-product of the biodiesel industry, on microbial nitrogen immobilisation and the soil microbial community. Soil was incubated with digestate, applied at a rate equivalent to 250 kg-N ha^-1, in a laboratory experiment over 50 days with glycerol additions at either 0, 12, 24 or 36 kg-C m³ of digestate. The addition of glycerol resulted in significantly higher microbial biomass carbon and increased the relative abundance of Gram-negative bacteria. The 24 and 36 kg-C m³ doses of glycerol resulted in similarly greater and longer lasting effect on microbial biomass carbon, indicating that beyond 24 kg-C m³ digestate that nitrogen (or other essential nutrients) became the limiting factor for microbial growth instead of carbon. Soil available nitrogen decreased throughout the study and remained at lower concentrations in glycerol treatments than the digestate only treatment by the end of the study. These results demonstrate that glycerol has the potential to reduce nitrogen losses from digestate application by immobilising nitrogen in the microbial biomass. Therefore, the co-application of digestate and glycerol to soil is a potential mechanism for the biogas and biofuel industries to valorise their respective by-products. Further research is needed to verify that this method is viable under field conditions.

Corrigendum: Immobilisation of anaerobic digestate supplied nitrogen into soil microbial biomass is dependent on lability of high organic carbon materials additives

Global food production relies on inorganic nitrogen to meet population consumption demands. However, synthetic fertilizer production currently relies heavily on fossil fuels. Anaerobic digestate offers an alternative source of nutrients because it is produced from organic matter. Yet digestate application results in similar problems of environmental pollution that synthetic fertilizer use incurs, such as nitrate leaching and N ₂ O emissions. Co‐amendment of digestate with a high carbon amendment may result in higher nitrogen use efficiency and fewer losses by adjusting the C:N ratio to encourage microbial assimilation of nitrogen. Therefore, we conducted a field plot experiment to determine the effects of adding carbon with digestate on the nitrogen uptake, yield, and quality of sugar beet ( Beta vulgaris ) grown on a calcareous sandy‐loam soil. N‐rich liquid digestate was applied at 100 kg‐N ha ⁻¹ , with carbon added as either glycerol or straw at 24 kg‐C m ⁻³ and compared against farm standard application of inorganic nitrogen. Sugar beet yield and quality under liquid digestate application performed comparatively with synthetic fertilizer, indicating that digestate can be used to fertilize sugar beet. The addition of glycerol resulted in higher microbial biomass carbon than digestate alone in the first month after application. However, no benefit to crop or nitrogen retention in soil was recorded by adding carbon to digestate, which could be due to the low carbon addition rate. Future studies need to focus on determining the optimal amount of carbon to add with digestate to positively impact yield and reduce nitrogen losses.