The role of humic substances in mitigating greenhouse gases emissions: Current knowledge and research gaps

Humic substances (HS) constitute a highly transformed fraction of natural organicmatter (NOM) with a heterogeneous structure, which is rich in electron-transferring functional moieties. Because of this feature, HS display a versatile reactivity with a diversity of environmentally relevant organic and inorganic compounds either by abiotic or microbial processes. Consequently, extensive research has been conducted related to the potential of HS to drive relevant processes in bio-engineered systems, as well as in the biogeochemical cycling of key elements in natural environments. Nevertheless, the increase in the number of reports examining the relationship between HS and the microorganisms related to the production and consumption of greenhouse gases (GHG), the main drivers of global warming, has just emerged in the last years. In this paper, we discuss the importance of HS, and their analogous redox-active organicmolecules (RAOM), on controlling the emission of three of the most relevant GHG due to their tight relationship with microbial activity, their abundance on the Earth's atmosphere, and their important global warming potentials: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). The current knowledge gaps concerning the microbial component, on-site occurrence, and environmental constraints affecting these HS-mediated processes are provided. Furthermore, strategies involving the metabolic traits that GHG-consuming/HS-reducing and -oxidizingmicrobes display for the development of environmental engineered processes are also discussed. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

This paper discusses the importance of humic substances and their redox-active organic molecules in controlling the emission of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), which are the main drivers of global warming. The study provides insights into the relationship between these substances and microorganisms related to greenhouse gas production and consumption. Additionally, it offers strategies for the development of environmental engineered processes involving GHG-consuming and HS-reducing/-oxidizing microbes' metabolic traits.