A widely recognized process, the biological carbon pump in marine systems, shuttles photosynthetically fixed carbon from the surface to the deep ocean through food web transfers and the sinking of dead organisms. The carbon in the deeper layers is ‘locked’ away, making the ocean a big carbon sink. An analogous biological pump has been postulated in soils, with microbial processing of plant organic matter locking up photosynthetically fixed carbon in the form of microbial residues, particularly into the deeper mineral layers.

Kallenbach et al.’s 2016 paper provided direct evidence for the formation of chemically diverse and stable soil organic matter in a clay–sand mixture with microorganisms fed on simple carbon substrates in the absence of plant inputs. The paper has led to a paradigm shift in our understanding of the formation and persistence of soil organic carbon. We now believe that most plant organic matter is decomposed by soil microorganisms on a relatively short timescale (months to years), whereas it is microbially processed organic matter (microbial dead residues, or necromass) that is more chemically recalcitrant and sticks to reactive minerals, allowing it to persist for much longer (decades).