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There is an urgent need for better management practices regarding livestock farm nutrient imbalances and for finding alternatives to the actual use of mineral fertilizers. Acidification of animal manure is a mitigation practice used to reduce ammonia emissions to the atmospheric environment during manure storage and land application. Acidification modifies manure physicochemical characteristics, among which soluble N and P significantly increase. The main objective of this study was to investigate if acidification and the addition of a nitrification inhibitor to manure and placement of the treated manure close to the seed can stimulate maize growth by enhancing nutrient availability, specially P and consequently plant P uptake, at early development stages without the use of mineral N and P as a starter fertilizer. Raw dairy slurry and solid fractions from dairy slurry and digestate from a biogas plant were acidified to pH 5.5 and applied with or without a nitrification inhibitor (DMPP, 3,4-dimethyl pyrazole phosphate) to maize in a pot experiment, where biomass productivity, nutrient uptake and soil P availability were examined. Acidification increased the water-extractable P fraction of all slurry and digestate organic residues (by 20–61% of total P) and consequently plant P uptake from solid fractions of both slurry and digestate compared to the untreated products (by 47–49%). However, higher plant biomass from acidification alone was only achieved for the slurry solid fraction, while the combination of acidification and DMPP also increased plant biomass in the digestate solids treatment (by 49%). We therefore conclude that the combination of acidification and a nitrification inhibitor can increase the starter fertilizer value of slurry and digestate products sufficiently to make them suitable as a maize starter fertilizer.

Pig slurry, rich in plant nutrients such as nitrogen (N) and phosphorus (P), is generally applied to soil as organic fertilizer. However, costs related to slurry transport may limit its utilization to fields close to the farm, leading to significant N losses, namely ammonia (NH3) emissions. Slurry acidification, to minimize NH3 emissions, is a potential solution to this problem, while solid-liquid separation leads to a solid fraction (SF) - rich in organic matter (OM) and phosphorus - and a liquid fraction (LF) rich in soluble nutrients. We hypothesized that a combination of acidification and separation could affect the quality of the resulting fractions depending on the separation technique used. After acidification, the two most common techniques for separation, centrifugation (CF) and screw-press (SP), were applied. The main characteristics of the slurry fractions in terms of nutrient concentrations and speciation as well as the potential N mineralization (PNM) were analysed. Our results show SFs with improved properties, mostly N and PNM when acidification is performed before separation with both techniques. The PNM was significantly increased in LFs from both techniques after acidification. The [Formula: see text] concentration increased in LFs from SP with acidification; therefore, slurry acidification is recommended to avoid any N losses during the separation process with SP, while CF may not require such pretreatment. Acidification could allow the use of a cheaper technique such as SP relative to CF since it prevents NH3 emissions during the separation process and leads to more equilibrated fractions in terms of nutrient composition.

Solid-liquid separation is performed to improve slurry management, and acidification of the slurry is used to reduce ammonia emissions. Acidification is known to affect slurry characteristics, and we hypothesized that it may affect mechanical separation. Our objective in this study was to assess the effects of slurry acidification on particle size distribution and separation efficiency. Two types of slurry, aged pig and fresh dairy, and two different acidification additives, sulfuric acid and aluminum sulfate (alum), were studied. We found that acidification with sulfuric acid promoted phosphorus (P) solubilization for both slurries, but no change was observed with alum. More ammonium was found in the acidified dairy slurry compared with raw dairy slurry, but no difference was found in aged pig slurry. Acidification before separation increased the proportion of the solid fraction in the slurries, and the effect was significantly higher with alum. When alum was used to acidify the slurries, the proportion of particles larger than 100 μm increased significantly, as did the P concentration in this particle size range. The efficiency of P separation increased markedly in both slurries when alum was used, with the removal to the solid fraction of the dairy slurry being almost complete (90%). Because the priority in mechanical separation is to increase the P content in the solid fraction, the use of alum before centrifugation may be the most suitable option for enhancing its nutrient content. We conclude that separation efficiency and particle size distribution are significantly affected by acidification, but the extent of the effects depends on slurry type and on the type of additive used for acidification.