This study used ceramsite and superphosphate as exogenous additives for composting garden wastes. It investigated the changes in temperature, pH, lignocellulose, enzyme activity, humic substrates, and humification parameters during the composting process. A total of four treatments—T0 (CK), T1 (ceramsite), T2 (superphosphate), and T3 (superphosphate + ceramsite)—were tested in a 50-day composting experiment. It was found that the formation of the compost humic substrate could be increased to 14.69% under the action of ceramsite. In addition, ceramsite promoted lignocellulose degradation, improved enzyme activity, and enhanced the material conversion in the composting process. At the same time, superphosphate also promoted the formation of humic substrates in compost but was less effective than ceramsite. Ceramsite is the best additive in promoting compost maturation.

With the rapid expansion of urban green spaces and the increasing amount of domestic waste, efficient and sustainable treatment of green waste (GW) and kitchen waste (KW) has become a pressing issue. Co-composting offers a green and low-carbon solution, yet a systematic understanding of its greenhouse gas (GHG) emission dynamics remains lacking. This study aims to investigate the impact of varying GW:KW ratios on GHG emissions during composting, in order to identify optimal mixing strategies and sup-port the development of low-carbon urban waste management systems. Six treatments with different GW:KW ratios (10:0, 9:1, 8:2, 7:3, 6:4, and 5:5) were evaluated under continuous aeration for 42 days. Results showed: (1) All treatments exhibited a typical composting temperature profile (mesophilic, thermophilic, cooling, maturation), with final seed germination index (GI) > 95% and significantly reduced E4/E6 ratios, indicating maturity. (2) When kitchen waste (KW) was ≤20%, cumulative GHG emissions slightly increased; KW ≥ 30% led to net reductions, with the 6:4 treatment (A4) achieving the highest decrease (17.44%) in total CO₂-equivalent emissions. In conclusion, maintaining KW at 40–50% optimally balances compost maturity and emission reduction, providing a viable strategy for the high-value utilization of urban organic waste and carbon mitigation.