Effects of different processes on gas emission during composting of cow manure

Composting manure is one of the most important ways to treat livestock manure in China. Large-scale breeding is an ever-increasing high demand for animal products, particularly with the rapid development of animal husbandry. A large number of discharged livestock manure has posed a serious risk to environmental protection. Composting has been widely used as an efficient technology for the resource utilization of manure. But there is some CO₂, CH₄, N₂O and NH₃ emission in the period of composting. The loss of material nutrients can reduce the fertilizer efficiency of compost products, leading to secondary pollution and the global greenhouse effect. The current treatment of cattle manure cannot fully meet the large-scale production, due to the simple composting in the low-scale of beef cattle farms. This study aims to explore the emission regularity and greenhouse effect of different composting treatments on greenhouse gases and ammonia. A 71 d experiment was also conducted on cattle manure composting in the Miyun District of Beijing, China. Cattle manure and corn stalks were selected as the raw materials. Four treatments were set (static composting, T1; composting + turning, T2; composting + film mulching, T3; turning + film mulching, T4). The results showed that the highest temperatures of T1, T2, T3 and T4 were 67.70℃, 70.13℃, 63.12℃, and 67.18℃, respectively, while the duration of high-temperature period (> 55℃) was 11, 11, 4 and 13 days, respectively. The initial moisture content of the four experimental groups was 70.01%, indicating a downward trend on the whole, especially in the high-temperature period. On the 14th day, the moisture contents of each treatment decreased by 30.92%, 29.25%, 35.94%, and 35.94% respectively. In the middle period of composting, the moisture contents of T1 and T4 treatments rebounded and increased, but they were still decreasing on the whole. At the later stage of composting, the moisture contents of each treatment were 29.66%, 15.63%, 37.05% and 41.38%, respectively, with decrease rates of 57.63%, 77.67%, 47.08%, and 40.89%, respectively. The germination indexes of the four treatments were 131.33%, 134.49%, 108.76%, and 136.24%, respectively, which all met the maturity requirements of compost products (≥70%). The turning promoted the temperature rise of the pile from the index of temperature and maturity. Both T2 and T4 treatments showed that the secondary temperature rose on the second day of turning. The heat preservation of the pile was improved by covering it with plastic film. All four treatments had rapidly raised the temperature for the high temperature to kill the harmful bacteria, thus making the pile harmless. The tipping point was beneficial to the emission of CO₂, N₂O and NH₃, according to the gas emission. The plastic film mulching also inhibited the generation of N₂O. The warming potentials of the four treatments were 23 573.33, 25 048.34, 18 826.63, and 24 680.09 g/(d·m²), respectively, at the end of composting. The warming potential of the T3 treatment was the lowest, while the emission reduction rate was 20.14%, compared with the T1 treatment. Therefore, T3 treatment can be expected to effectively reduce the warming potential to the maturity of compost products. This finding can provide technical guidance to optimize the practice selection of composting in small and medium-sized farms.