Microbial solutions to soil carbon sequestration

Inoculation of soil with living microbes or propagules has grown in interest and in application due to the modification/degradation of soil systems (including native microbial communities), and the need to maintain agricultural yields with fewer synthetic inputs. However, whilst beneficial microorganisms such as plant growth promoting bacteria (PGPB) and mycorrhizal fungi have been employed via inoculation, their utilisation as an agronomic tool remains trivial in the context of large-scale commercial agriculture. The development of inoculation products has thus far largely focused on their capacity to support plant health (and correlating yield/ profitability), with little attention paid to the ability of these organisms/products to influence soil carbon. Given the expected growth of the agricultural inoculant industry (estimated to reach US$12.5b in revenue by 2027), the increasing commercialisation of soil carbon sequestration (via carbon credits and other financially linked instruments), and the need to find viable solutions to assist in the drawdown of atmospheric CO2, a greater understanding of the role of soil microbes in soil carbon cycling is required in order to facilitate the development of products capable of supporting the sequestration and retention of soil carbon.Here, we review the mechanisms by which microorganisms contribute to soil carbon sequestration and retention and suggest several groups that may be promising candidates for further exploration. Of the many microbial mediated mechanisms identified, we highlight (among others) the capacity of arbuscular mycorrhizal fungi to facilitate the transition of carbon from labile to recalcitrant pools (mineral associated and aggregated), melanising endophytic fungi as a potential source of stabile soil carbon, and PGPB as stimulators of plant growth/ reliance and thus carbon entering the soil carbon pool. We put forward the 'biochar + microbe system' as a potential avenue to overcome the current limitations to building and retaining soil carbon stocks. This review is timely, given the challenges facing global food production, and the need to find viable solutions to address climate change.