Loss of nitrogen (N) via ammonia (NH₃) volatilization during composting of livestock manure causes nutrient imbalance in the compost. The effects of phosphogypsum (PG) and zeolite amendments on reducing N loss (via suppressing NH₃ volatilization) and concurrent changes in carbon (C) and dry matter losses were investigated in a 100-day lab-scale mesophilic (27 °C) composting of cattle manure with rice hulls or sawdust as a bulking agent. The results showed that amendments PG and zeolite did not reduce NH₃ volatilization but decreased total N loss, and when combined with sawdust the N loss was further reduced. However, the effects of PG and zeolite on reducing N loss were neither additive nor synergistic, and thus co-amending PG and zeolite did not further reduce N loss. The C and dry matter losses were smaller when sawdust was used as a bulking agent compared to rice hulls, and PG and zeolite amendments also decreased the C and dry matter losses in the presence of bulking agents. Combining sawdust as a bulking agent with either PG or zeolite amendments was more efficient than other combinations in reducing N, C, and dry matter loss in mesophilic composting of cattle manure.