Sulfur in humin as a redox-active element for extracellular electron transfer

•Sulfur is a redox-active element in humin for extracellular electron transfer.•Oxidization states of sulfur in all humin ranged widely from −2 to +6.•Sulfur with oxidation states from −2 to +5 in the organic fraction supports humin function.•Sulfur plays important role in microbial network in geosphere.•Distribution of sulfur oxidation states in humin reflects the source environment.