Effects of salt content on the humification and microbial community succession of food waste aerobic composting

Food waste is ever increasing in rural areas in recent years, particularly with the rapid economic development and population growth. If disposed of improperly, severe pollution can pose a great threat to the rural living environment. Fortunately, aerobic composting has been widely applied for food waste treatment, due to the high harmlessness and resource utilization rate. It is worth noting that food waste with high salinity can limit the application of aerobic composting. Among them, humification is one of the most important processes in the application of compost. However, only a few studies focused on the effect of salt content on aerobic compost humification. It is still lacking in the influencing mechanisms from the perspective of microbial community structure evolution. Taking food waste as the main raw material, and the maize stover as an auxiliary material, this study aims to explore the influences of salt content on the basic maturity index, organic matter composition, and dynamic change of humus substances during the composting process. The succession of microbial community was combined with the correlation statistical analysis, in order to reveal the microbial action about the effect of salt content on the mature process of food waste aerobic composting. Specifically, the food waste and maize stover were mixed well at the ratio of 4:1, where the initial moisture content was adjusted to about 63%. A composting experiment was carried out in the 60 L composting reactors for 45 days. A total of four experimental treatments were set with the addition of 0%, 0.5%, 1.0%, and 1.5% salt. Among them, the treatment without salt was the control group. The results showed that the temperature, C/N, and organic matter composition of four treatments fully met the requirements of maturity during aerobic composting. However, the salt addition greatly increased the electrical conductivity of the reactor. Particularly, the thermophilic period was shortened to 13 days at the proportion of salt addition of 1.5%. But, the seed growth was inhibited with the seed germination index reducing to 65.5%. Compared with the control treatment without salt, the degradation rate of organic matter was reduced by 6.5%, indicating the restricted humification process. The high throughput sequencing showed that the addition of 1.5% salt inhibited the activities of Thermobifida, Saccharomonospora, Aspergillus, and Trichosporon. In addition, a correlation analysis showed that the high salinity reduced the biodegradation of organic matter, such as total sugar and lignocellulose, thereby limiting the subsequent formation of humus substances. Thus, restricted improvement was found in the humification effect during food waste and maize stover composting. These findings can provide theoretical support for salt regulation in the application of aerobic composting in food wastes.