Fertiliser value and GHG emissions from solid fraction pig slurry compost pellets

Conversion of pig slurry to pellets is a desirable fertiliser option for farmers who want to mitigate environmental pollution from slurry accumulation. The goals of this investigation were to determine the fertiliser properties of pig slurry solid fraction (SF) pellets, and to assess its potential to enhance soil properties in order to reduce NH3 volatilisation and GHG emissions. Various parameters influence SF-based pellet fertiliser effectiveness: bulking agent use during composting, pellet diameter sizing, and soil application type (superficially or incorporated into the soil). Here, we prepared two composts from the same pig slurry solid fraction obtained from a screw press separator: pig SF compost without a bulking agent (SSFC) and pig SF compost with wood chips as the bulking agent (WCC). For each compost type, pellets of two different diameters (6 mm and 8 mm) were produced. A mesocosm experiment, conducted with maize plants, was used to test the fertiliser value of the considered pellets. In total, three compost fertilisers--SSFC, WCC, and NPK mineral fertiliser 15:15:15, plus one unfertilised control treatment were applied at the same N rate (equivalent to 200 kg ha-1) using two different methods (surface and soil incorporation). After 65 days, aboveground biomass, roots, and soil samples were collected and analysed. Subsequently, a second mesocosm study was undertaken to measure NH3 and GHG emissions released from pellet fertilisation. Ammonia volatilisation was determined immediately after pellet application, while CO2, CH4, and N2O emissions were monitored for 57 days. Study results indicated that both pellet types were effective slow-release fertilisers for maize. Additionally, three actions seemed to make the nutrients contained in pig SF compost pellets more available to plants: addition of a bulking agent before composting, use of small diameter pellets, and soil incorporation of the fertiliser.