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 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.

Bioacidification of animal slurry has proven to be a good alternative to traditional acidification with sulfuric acid for reducing ammonia emissions. However, the fertiliser value of the bioacidified slurry is yet to be determined before a whole-system assessment can be made. The N fertiliser value of pig slurry either untreated or bioacidified with glucose and/or fermented brown juice (BJ) was investigated in a pot experiment with maize (Zea mays L.) grown in a greenhouse. The slurry treatments were either pre-acidified with sulfuric acid to pH 6.5 or 5.5 before bioacidification, or bioacidified without pre-acidification. Plant growth was good in all treatments, but the bioacidified treatments showed a lower mineral fertiliser equivalence (MFE) value than the non-bioacidified treatments. Average MFE values were 71 %, 62 %, 59 % and 41 % for the non-bioacidified (noC), glucose (glu), glucose and brown juice (glu + BJ20) and brown juice (BJ50) treatments respectively. This reduction was most likely caused by immobilisation of N due to the addition of easily available C from the substrates. The fertiliser value was not affected by the pH, C/N ratio and C content of the treatments, while a positive correlation was found with NH4 + -N content. Pre-acidification positively affected MFE, probably due to higher nutrient availability. Further studies on the effect of different inclusion rates of substrates for bioacidification and the effect of application method on the fertiliser value, as well as studies under field conditions, are needed before recommendations can be made about bioacidification as an alternative to traditional acidification.