Impact of manure anaerobic digestion and acidification of digestate on soil quality and GHG emissions

Acidification of stored manure and on‑farm anaerobic digestion have been identified in Canada as beneficial management practices (BMPs) that can mitigate climate change by reducing greenhouse gas (GHG) emissions. The objective of this study was to assess the effects of these technologies on manure agronomic value, soil physicochemical properties, and nitrous oxide (N₂O) emissions.

Field experiments were established in 2024 at two contrasting sites classified as a silty clay loam and a sandy loam. Five treatments were applied in a randomized design with three replicates per site: undigested manure, digestate, acidified digestate (pH 6.5), mineral fertilizer, and an unfertilized control for a total of 30 plots. During the growing season, soil samples (0–20 cm) were analyzed for pH, major nutrients (N, P, K, Ca, Mg, S), and trace elements (Cu, Zn, Fe, Mn, Al). Treatments were applied in spring and after forage harvests. Before each application, manure samples were analyzed for density, pH, and total major and trace element concentrations.

To quantify N₂O emissions, non–steady‑state chambers were installed in spring and remained in place throughout the season. Chambers were sampled weekly, and gas concentrations were measured using a gas chromatograph with an electron capture detector. Forage yield was measured  for each plot, and plant tissues were analyzed for total major and trace elements using ICP‑OES.

Soil pH ranged from 5.8 to 6.6 in the silty clay loam and from 5.9 to 6.8 in the sandy loam. Manure‑based treatments emitted 1.2 to 3.9 times more N₂O than the mineral fertilizer treatment, except in the silty clay loam in 2024, where manure treatments (excluding the control) produced similar emissions. The acidified digestate treatment consistently generated the highest N₂O emissions among manure treatments.

In sandy loam plots, N₂O production was 1.3 and 2.6 times higher in the undigested manure and acidified digestate treatments, respectively, compared with the silty clay loam. Cumulative N–N₂O losses were also greater in 2025, particularly for the acidified treatment (73% higher), while increases in the control and mineral fertilizer treatments were more moderate (28% higher).

Forage yields were higher in 2025 than in 2024. In the silty clay loam, yields followed the order: digestate > acidified > mineral > undigested > control, ranging from 1.8 Mg ha^-1 (control) to 2.8 Mg ha^-1 (digestate). A similar trend occurred in the sandy loam, where the acidified treatment produced the highest yields, followed by the digestate.

Overall, the results show that treated and acidified manure applications improved forage yield relative to mineral fertilizer in both soils. However, the acidified treatment also increased N₂O emissions, suggesting that its reduced pH may require a longer stabilization period before field application. This aspect will be examined in the coming years. Additional project data will also help identify BMPs that can effectively reduce N₂O emissions on Canadian farms.