Laccase Promotes the Formation of Fe-OM Complexes by Catalyzing the Polymerization of OM in Coastal Saline-Alkaline Soil

Laccase is critical in the formation of iron (Fe)-organic matter (OM) complexes in soil, which plays a key role in the stabilization and storage of OM. However, the formation mechanism of Fe-OM complexes mediated by laccase in coastal saline-alkaline soil is still unclear. A cultivation experiment was arranged with 2 factors, in which the pH was 7.5, 8.0 and 8.5, respectively, and the carbon (C) /Fe molar ratio was 10, 5 and 2, respectively. When pH was 7.5 and the initial C/Fe molar ratio was 2, the removal rates of dissolved organic matter (DOM) and dissolved Fe could reach to 28.8% and 77.8%, respectively. Meanwhile, gel permeation chromatography analysis showed that laccase could catalyze the polymerization of DOM by increasing the molecular weight of DOM, which was more conducive to combine with Fe and form Fe-OM complexes in saline-alkaline soil. Fourier transform infrared spectroscopy showed that the OM combined with Fe catalyzed by laccase contained more oxygen-containing functional groups, but less C = C bonds. The X-ray diffraction and X-ray photoelectron spectroscopy showed that, the species of Fe in the two systems were polycrystalline Fe phase, and FeOOH, Fe (II) and Fe (III) coexisted on the surface of Fe-OM complexes in both laccase and laccase-free system. Laccase could change the structure of OM and promote the formation of Fe-OM complexes by catalyzing the polymerization of OM. Moreover, laccase could also increase the C/Fe molar ratio of Fe-OM complexes, which improved the sequestration of OM in coastal saline-alkaline soil.