Abstract: Abstract: Composting is an inexpensive and sustainable treatment for biogas residues. Therefore, assessment of compost maturity is crucial for achieving high quality compost in order to guarantee its marketability. In this context, synchrotron-radiation-based infrared spectromicroscopy (SR-FTIR) was developed to in situ characterize changes of functional groups at particle scale during composting of biogas residues, which were more suitable for assessment of compost maturity than the conventional techniques in view of ease of sample preparation, rapid spectrum acquisition, and nondestructive nature of the analysis. Intact biogas residue particles (100-500 μm) were picked using superfine tweezers from samples before and after composting. The selected particles were then frozen at ?20 ℃ and directly sectioned without embedding. Thin sections (2 μm in thickness) were cut on a cryomicrotome and transferred to infrared-reflecting MirrIR Low-E microscope slides and then observed at beamline BL01B1 of the National Center for Protein Science Shanghai (NCPSS). The results showed that all of the peaks at 2 930, 1 721, 1 436, 1 234, 1 020 and 866 cm(1 were presented in biogas residue particles before and after composting, suggesting lipids (absorbance at 2 930 cm(1), carboxylic acids (absorbance at 1 721 cm(1), lignin (absorbance at 1 436 cm(1), and carbohydrates (absorbance at 1 234, 1 020 and 866 cm(1, respectively) as the main components of biogas residues. During anaerobic digestion, most of lipids in biogas residues were degraded, but only few of carboxylic acids and lignin were degraded and carbohydrates enriched in CO were formed. Furthermore, micro-FTIR spectra showed the spatial changes of organic functional groups in biogas residue particles. Compared to raw biogas residues, the peaks at 1 721 and 1 234 cm(1 were disappeared after composting and formed 2 new peaks at 1 715 and 1 156 cm(1, which might relate to humification process and compost maturity in the composting process. In addition, correlation analysis between organic OH (absorbance at 3 400 cm(1) and aliphatic C (absorbance at 2 930 cm(1), carboxylic acids (absorbance at 1 721 cm(1), and carbohydrates (absorbance at 1 030 cm(1) was conducted in biogas residue particles before and after composting. A strong linear correlation among them was consistent with the results of synchrotron-radiation-based infrared mappings, confirming their co-degradation during composting. However, the distribution of functional groups in biogas residues after composting was more heterogeneous than that before composting. Taken together, synchrotron-radiation-based infrared spectromicroscopy provided information about the distribution pattern of functional groups in the biogas residue particles, while micro-FTIR spectra showed spatial changes of functional groups within biogas residues. In summary, synchrotron-radiation-based infrared spectromicroscopy is expected to be a new method to characterize compost maturity during composting of biogas residues.