Fertilizer value and greenhouse gas emissions from solid fraction pig slurry compost pellets

Conversion of pig slurry to pelletized fertilisers is a desirable option for farmers in order to prevent environmental pollution, related to slurry accumulation. The aim of the present work was to determine the fertilizer properties and the potential benefit for improving soil properties and reducing NH3 volatilization and GHG emissions of pelleted pig slurry solid fraction (SF). Different techniques for producing SF-based pelletized fertilizer have been proposed, based on composting with the addition of a bulking agent, preparation of different pellet sizes and using different methods of soil application (superficial and mixed with the soil). Two composts were prepared from the same initial pig slurry solid fraction obtained from a screw press separator: pig SF compost without bulking agent (SSFC) and pig SF compost with wood chips as a bulking agent (WCC). For each compost investigated, two different types of pellets with diameter of Ø 6 mm and Ø 8 mm were made. A mesocosm experiment, carried out with maize plants, was set for testing the fertilizer value of the investigated pellets. Three fertilizers SSFC, WCC, and NPK mineral fertilizer (15:15:15) at the same N application rate equivalent to 200 kg ha-1 were tested and compost were applied at both the surface or mixed with the soil. Unfertilized control treatment was also included. After 65 days, aboveground biomass, roots and soil were collected and analyzed. A second mesocosm study was carried out for the determination of NH3 and GHG emissions caused by pellets fertilization. Ammonia volatilization was determined immediately after pellets application, while emissions of CO2, CH4 and N2O were monitored for the following 57-day period. The results of this study highlighted that both pellets investigated acted as effective slow-release fertilizers for maize. The best technical options for their production and application include the use of a bulking agent before composting, the choice of a small diameter for pelletizing, and the incorporation into the soil (this last inducing an increase in N2O emissions). All these techniques improves the availability of nutrients in pelletized compost from pig SF for plant nutrition.