Integrating glycerol and straw into digestate applications: Microbial biomass and crop responses under field conditions

Abstract 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.