Effect of semi-permeable membrane coverage on carbon dioxide emissions during composting: Insights into carbon metabolism pathways, dominated by the tricarboxylic acid cycle.

The tricarboxylic acid (TCA) cycle is the main carbon metabolism pathway leading to carbon loss in the form of carbon dioxide (CO2). Research on the CO2 emission reduction mechanism based on the TCA cycle under different composting strategies is relatively lacking. This study investigated the effects of semi-permeable membrane coverage on CO2 emissions and pathways of carbon metabolism during three batches of large-scale dairy manure aerobic composting by using gas chromatography and high-throughput sequencing technologies. The results showed that the CO2 emission rate peaked at the initial stage of composting and stabilized after the 10th day. Among all groups, the semi-permeable membrane coverage coupled forced aeration groups had the lowest CO2 emission rate. Using semi-permeable membrane coverage during aerobic composting increased the abundance of Kyoto Encyclopedia of Genes and Genomes Ortholog (KO) K01647, citrate synthase, but decreased the total abundance of KOs related to the TCA cycle. KOs K01895 and K01638, acetyl-CoA synthetase and malate synthase, respectively, played a regulatory role in the intensity of the TCA cycle during aerobic composting; and their abundances were lower in the semi-permeable membrane-covered compost groups than in the traditional static compost groups. Therefore, citrate synthase was not a limiting factor for the TCA cycle during aerobic composting, and acetyl-CoA synthetase and malate synthase played a regulatory role in the intensity of the TCA cycle. In sum, using semi-permeable membrane coverage in the aerobic composting process of dairy manure is an effective strategy to decrease carbon loss by inhibiting the TCA cycle.