Mechanistic study on laccase-mediated formation of Fe-OM associations in peatlands

Abstract Peatlands represent one of the largest terrestrial carbon pools. Phenol oxidase is widely considered to play a critical role in the decomposition of organic matter in peatlands, which is known as the “enzymatic latch”. However, our recent studies have indicated that the formation of iron-organic matter (Fe-OM) associations is more likely to occur in the presence of laccase, the largest class of phenol oxidases in soils. As the formation of Fe-OM associations is recognized as an important stabilization mechanism for soil organic matter, further clarification of the potential mechanisms contributes to a better understanding of the role of laccase in peatland carbon sequestration. In this study, a series of experiments using different phenolic compounds and peat-derived dissolved organic matter (DOM) were performed. The results from liquid chromatography–high-resolution mass spectrometry (LC-HRMS) and gel permeation chromatography (GPC) analyses indicated that laccase catalyzed the polymerization of phenolics and peat-derived DOM by coupling of the C-O bond, the C–C bond and the C–C/C-O bonds. The polymerized products can combine more effectively with Fe to form Fe-OM associations. FTIR spectral analysis indicated that organic matter associated with Fe under laccase catalysis contained more oxygen-containing functional groups but fewer aromatic C C bonds. Although laccase catalysis can change the chemical structure of organic matter, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed minimal influence of laccase on the Fe chemical species in the Fe-OM associations. Thus, we propose that laccase mediated the formation of Fe-OM associations by polymerizing organic matter and by increasing the O/C ratio of organic matter. In addition, laccase can improve the sequestration of organic carbon by increasing the C/Fe molar ratio of the formed Fe-OM associations. This mechanism may provide further insight into the role of laccase in the carbon cycling in peatlands.