Effects of sepiolite addition on pig manure compost maturity and dissolved organic matter

Pig manure has caused the most serious environmental pollution among various animal manure, where estimated approximately 776 million tons in each year in China. Aerobic composting can be expected as an effective technique to treat the solid organic wastes, thereby to decompose inconstant and hazardous organic matter, and futher to quickly reduce the total amount and inactivate biotoxicity of wastes. Previous reports indicated that clay minerals have observably influence on the decomposition of Organic Matter (OM) in soil system. However, the research is still lacking on the intermolecular interactions between clay minerals and OM in the composting, even though the OM was more simple and active. Taking the pig manure and poplar sawdust as raw materials, and sepiolite as a conditioner, this study aims to explore the influence of sepiolite on the stability of aerobic composting. An investigation was made on the variation in the maturity index of compost, organic matter in the different components of compost, and structure of Dissolved Organic Matter (DOM). The results showed that after sepiolite added, the maximum temperature of compost decreased obviously, and the electrical conductivity value increased by 9.69%, compared to control. However, the lower C/N (decreased by 2.81%) and higher seed germination index (increased by 11.96%) were observed with the addition of sepiolite without the negative impact of finial production, while showing better maturity. These indicators suggested that the organic fertilizers with the sepiolite addition were beneficial to the application for the farmland. Most previous studies focused on the content of DOM and humic acid, representing the stable and unstable components of OM in the compost production. Compared with the control, DOM content of compost with the addition of sepiolite was reduced by 7.84%, while the percentage of humic acid increased by 9.71%, indicating that the sepiolite can influence on the content of different components of OM, and thereby make the compost more stable. In this study, fluorescence spectra were used to represent the fluorescence characteristics of DOM, further to clarify the interactions between clay minerals and OM. An Excitation-Emission Matrices-Parallel Factor Analysis (EEM-PAFARAC) was used to quantify the proportion of DOM components. The results demonstrated that the sepiolite significantly increased the fluorescence intensity of long-wavelength peak in the fluorescence spectrum in a relatively short period, meaning that the more stable OM was produced more quickly. After the DOM components were distinguished by EEM-PAFARAC, the proportion of highly aromatic components increased significantly in the begining phase of compost with the addition of sepiolite, indicating more higher proportion in the final production. In order to explore the causes of OM transformation in composting, the correlation analysis showed that there was a more significant negative relationship between the highly aromatic component of DOM and total organic matter, compared with the control, indicating that the OM cannot decomposed, but converted into more stable OM in the sepiolite treatment. Therefore, the sepiolite as an additive can be used to reduce the biotoxicity of composting products, while to increase the degree of maturity, and the stability of compost via impacting on the structure of organic matter.