Effect of Adding Flue Gas Desulphurization Gypsum on the Transformation and Fate of Nitrogen during Composting

The objective of this study was to investigate the effect of adding flue gas desulphurization gypsum (FGDG) on the transformation and fate of nitrogen during co-composting of dairy manure and pressmud of a sugar refinery. The ammonia absorption of FGDG was investigated. The changes in compost temperature, pH, electrical conductivity (EC), moisture, organic matter, the C/N ratio, Kjeldahl N, NH₄⁺-N, NO₂^?-N, NO₃^?-N were assessed. The addition of FGDG did not significantly affect compost temperature, pH, EC, moisture, and organic matter degradation. However, the addition of FGDG significantly increased the NH₄⁺-N content in the compost during the thermophilic phase, and the NH₄⁺-N maximal content in the compost with FGDG (CP+G) was 59.9% more than that in the compost without FGDG (CP–G). FGDG was thought to create the formation of (NH₄)₂SO₄ and the cation exchange between NH₄⁺ and Ca²⁺. The NO₂^?-N content in the CP+G peaked on day 15, and was not observed in the CP–G. In the final compost products, the NO₃^?-N concentration in the CP–G was more than that in the CP+G, which was 1451 (CP–G) and 1109 mg·kg^?1 (CP+G) dry material. This might be due to the NO₂^? accumulation in the CP+G, which accelerated N loss in the form of N₂O. There is a strong correlation between N₂O emission and NO₂^?-N accumulation in the composting process. Compared with the original N content in the compost mixture, the N loss in CP–G and CP+G were 15.0 and 10.8%, respectively. These results revealed that NH₄⁺-N conservation effect was improved during the thermophilic phase and the total N loss was mitigated by adding FGDG into composting materials. FGDG could be utilized as a potential amendment to conserve nitrogen during composting.