Impact of biochar combined with earthworm treatment on nutrient transformation and retention in composting

High-temperature composting and vermicomposting have been the main means of value-added and resource utilization of agricultural wastes. However, the physical and chemical properties of the substrate can dominate in the final product of vermicomposting, vermicompost, as well as the growth and reproduction of earthworms during composting. High-temperature composting is often used as a pretreatment for vermicomposting to improve the palatability and conversion rate of the substrate. Biochar, with its well-developed porous structure and abundant surface functional groups, has been one of the most important exogenous additives to reduce nutrient loss during compost maturity. Furthermore, biochar can provide colonization sites for microorganisms in the intestines of earthworms, further facilitating the earthworms' absorption of nutrients from the substrate. This study aims to reduce nutrient loss during composting and vermicomposting at high temperatures, particularly with the high conversion rate of base materials, as well as the weight gain and reproduction of earthworms. Different proportions of biochar (0, 5%, and 10%) were added to chicken manure and corn stover for composting. Eisenia Fetida was used as the base material for 35 days of vermicomposting. A systematic analysis was conducted to explore the total nutrients, available nutrients, trace elements, humus components, and growth and reproduction of high-temperature composting and vermicompost. Scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), and three-dimensional fluorescence spectroscopy (EEM) were used to characterize the pre-composting and vermicompost. Microstructure, elemental composition, and functional groups of feces were obtained to evaluate the performance. The results showed that the large particles on the substrate surface were converted into tightly connected small granular substances, indicating a large number of pores after earthworm transformation. The addition of biochar treatment reduced the surface area and total pore volume, while increasing the average pore diameter. The pH value and conductivity of composting promoted the humification of the base material due to the high content of abundant and trace nutrients. Compared with high-temperature composting, there was a decrease in the total nutrients, humus, ammonium nitrogen, available phosphorus, and available potassium content of each treatment after vermicomposting, while the nitrate nitrogen content increased significantly. Humic acid was further converted to fulvic acid. FTIP, XRD, and EEM indicate that biochar was added to promote the degradation of aromatic compounds and lignin in earthworm compost, with an increase in polysaccharides. In addition, the total earthworm biomass, mass of a single adult earthworm, survival rate, and cocoons of earthworms treated with biochar were significantly (P<0.05) higher than those of the control group (CCM0), contributing to the retention of nutrients in vermicompost. Among them, the total content of alkali-hydrolyzed nitrogen, available phosphorus, and available potassium increased by 12.81% and 143.56%, respectively, after vermicompost treatment with 5% biochar (CCM1) and 10% biochar (CCM2), compared with the control group (CCM0). Therefore, the addition of biochar increased the nutrients after composting, while effectively reducing the nutrient loss after vermicomposting, according to the combined high-temperature composting and vermicomposting. The findings can provide a new approach for the weight gain and reproduction of earthworms, particularly for the high-quality vermicompost.