Effect of cattle slurry acidification on nutrient and carbon dynamics in the soil-plant system

Abstract

The pressure to mitigate gaseous emissions, including ammonia (NH₃) and greenhouse gases (GHG), has driven agricultural stakeholders to explore sustainable manure management practices such as slurry acidification. Acidifying cattle slurry with sulfuric acid (H₂SO₄) to pH 5.5 has demonstrated efficiency in reducing NH₃ emissions from animal housing, manure storage, and field applications. Moreover, H₂SO₄ enhances nitrogen (N) use efficiency, increases phosphorus (P) solubility in soils, and supplies sulfur (S) to deficient agricultural soils, thus improving crop yield and reducing environmental impact. While this approach shows potential as a sustainable practice, more research is required to assess its effects on nutrient cycling, soil-plant interactions, and broader ecosystem impacts over both short and long-term periods.
This study investigates the effects of both non-acidified (pH 6.5–7 for whole slurry, pH 8–9 for digestate) and acidified slurry (pH levels of 6.5, 6.0, 5.5, and 4.5) on the dynamics of N, phosphorus P, carbon (C), and S across various agricultural systems, including soil, crops, slurry storage, and field applications.
During storage, a higher dose of H₂SO₄ was required to acidify digested cattle slurry compared to undigested slurry to achieve target pH levels of 6.5, 5.5, and 4.5. Although Danish guidelines recommend low-dose acidification targeting pH levels above 5.5, we observed that pH 6.5 and 5.5 treatments resulted in pH rebound over five months storage, which would require re-acidification to maintain NH₃ emission reductions. Acidification to pH ≥ 5.5 before storage also led to increased sulfide formation in storage of whole, digested, and separated slurries.
In laboratory-scale incubation experiments, the surface application of acidified slurry (pH 5.5) did not significantly reduce C turnover from soil organic matter (SOM), but enhanced C turnover from added ryegrass residues (GCR), increasing by approximately 10% relative to non-acidified slurry. Acidified slurry application also led to a significant positive priming effect of GCR to approximately 14.3% compared with soil without any slurry application, suggesting enhanced chemical decomposition of plant material, likely increasing substrate availability for the soil microbial biomass (SMB).
Acidified slurry effectively reduced GHG emissions, particularly CO₂, with isotopic ¹⁴C labelling revealing a 50% reduction in SMB-derived CO₂ emissions compared to conventional slurry. However, unlike some reports, acidified slurry did not inhibit nitrification; instead, it increased the mineralization of C and N, raising soil N content without reducing N₂O emissions compared to conventional slurry.
Contrary to expectations, P availability was not enhanced by acidified slurry application in soils with a significant pH reduction. In a pot experiment with maize, acidification to pH 6.0 sufficed to meet crop N and P requirements. However, with ryegrass grown on a different soil texture, P availability increased with slurry acidification up to pH 5.5 but declined at pH 4.5. Ryegrass yield peaked during the first of four harvests, with decreasing N and P availability likely limiting subsequent yields.
Sulfate (SO₄²⁻) added through acidification remained stable after slurry storage, with negligible losses to sulfide formation. For oilseed rape, slurry acidified to pH 5.5 provided adequate S, as did pH 6.5 for digestates. The requirement for S in maize was met with slurry acidified to pH 6.0, while ryegrass required pH 6.5, with S uptake after four harvests representing < 10% of the supplied S.
While slurry acidification to pH 5.5 offers some environmental benefits, it is not a universally applicable solution. Each agricultural context presents unique requirements and challenges, including potential trade-offs in nutrient availability and pollution swapping. Future research should focus on field studies to optimize slurry acidification tailored to specific crops, soils, and environmental conditions, aiming to maximize benefits while minimizing unintended impacts on C turnover, N and P availability, and the soil S balance.

Date of Award	2025
Original language	English
Sponsors	Bangor University
Supervisor	Davey Jones (Supervisor) & Dave Chadwick (Supervisor)