Greenhouse Gas Emissions and Growth of Wheat Cultivated in Soil Amended with Digestate from Biogas Production

Digestate, the product obtained after anaerobic digestion of organic waste for biogas production, is rich in plant nutrients and might be used to fertilize crops. Wheat (Triticum spp. L.) was fertilized with digestate, urea, or left unfertilized and cultivated in the greenhouse for 120 d. Emissions of greenhouse gasses (carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)) were monitored and plant growth characteristics were determined at harvest. The digestate was characterized for heavy metals, pathogens, and C and N mineralization potential in an aerobic incubation experiment. No Salmonella spp., Shigella spp., or viable eggs of helminths were detected in the digested pig slurry, but the number of faecal coliforms was as high as 3.6 × 10⁴ colony-forming units (CFU) g⁻¹ dry digestate. The concentrations of heavy metals did not surpass the upper limits established by US Environmental Protection Agency (EPA). After 28 d, 17% of the organic C (436 g kg⁻¹ dry digestate) and 8% of the organic N (6.92 g kg⁻¹ dry digestate) were mineralized. Emissions of CO₂ and CH₄ were not significantly affected by fertilization in the wheat-cultivated soil, but digestate significantly increased the cumulative N₂O emission by 5 times compared to the urea-amended soil and 63 times compared to the uncultivated unfertilized soil. It could be concluded that digestate was nutrient rich and low in heavy metals and pathogens, and did not affect emissions of CH₄ and CO₂ when applied to a soil cultivated with wheat, but increased emission of N₂O.