Biochemical and microbial characterization of a forest litter-based bio-fertilizer produced in batch culture by fermentation under different initial oxygen concentrations.

This work focused on the physico-chemical, biochemical and microbiological characterization of a new organic fertilizer based on fermented forest litter (FFL) mixed with agro-industrial by-products, on the culture realized in airtight glass bottle. Under strict anaerobiosis (0% initial oxygen concentration (IOC)), after a 16-day batch culture, the bottle-headspace analysis showed that the specific CO₂ production rate was low (0.014 mL/h.g dry matter) compared to those reached under aerobic conditions (e.g. 0.464 mL/h.g dm at 21% IOC). Moreover, the culture displayed a slight fermented fruity odour, mainly due to ethanol and ethyl acetate detected in the headspace (335 µL and 58.6 µL accumulated, respectively). The FFL organic matter degradation followed by infrared spectroscopy and catabolic potential and diversity characterized by BIOLOG^® EcoPlates were poor and pH dropped to 4.54. The microbiome's metabolism was oriented toward lactic fermentation with medium acidification, enrichment in lactic acid bacteria (LAB), depletion in fungi and absence of pathogens. By increasing IOC from 0 to 21%, the respirometric activity, and the catabolic potential and diversity increased. However, some enterobacteria were detected above 5% IOC. Ethanol and ethyl acetate decreased strongly with IOC, and aromatics and proteins contained in the solid matrix remained in the culture. This study showed the importance of oxygen on the final product. A 2% IOC was found to ensure an optimal balance between LAB development, preservation of functional catabolic diversity and bio-product free of microbial pathogens.