Microbially reducible extent of solid-phase humic substances is governed by their physico-chemical protection in soils: Evidence from electrochemical measurements

A significant fraction of humic substances (HS) in natural soils is protected due to the interactions with mineral surfaces or occlusion within aggregates and clay microstructures, which could render the solid-phase HS less spatially accessible to microorganisms and thus affect their extracellular electron transfer. However, more diverse and convincing evidence is needed to further verify the link between extracellular electron transfer of solid-phase HS and their physico-chemical protection mechanisms in soils. In this study, soil physical fractionation and mediated electrochemical measurements were combined for the first time to assess the microbially reducible extents of solid-phase HS in mineral soils. The results show that the solid-phase HS in soil contain several pools with different microbially reducible extents, due to the fact that different pools employ distinct protection mechanisms of soil organic matter against microbial attacks; the particulate HS exert the greatest microbially reducible extent, followed by the microaggregate occluded HS and non-aggregated silt associated HS, and non-aggregated clay associated HS exert the lowest microbially reducible extent. Our work provides a new evidence to validate the view that the extracellular electron transfer process of solid-phase HS in natural soils is largely controlled by their physico-chemical protection mechanisms. (C) 2019 Elsevier B.V. All rights reserved.