This review examines “the state of knowledge for the stocks of, inputs to, and outputs from SOM around the world” [Jackson 2017: 422], with a view toward developing better understanding of processes that stabilize SOM. It explains the biological processes involved in carbon cycling and storage, finding that “root inputs are approximately ﬁve times more likely than an equivalent mass of aboveground litter to be stabilized as SOM” [Jackson 2017: 420]. Litter input can either increase or decrease SOM, despite the assumption in most carbon models that there is a linear relationship between litter input and transformation of carbon into more stable forms. This finding suggests that perennials and other deep-rooting plants have an important role to play with respect to carbon sequestration. As the author puts it:
Managing carbon inputs and relative allocation, for instance, through selection for deep roots or for greater belowground allocation in crops (Kell 2011), has been suggested as a way to increase SOM formation and stabilization in such systems (Bolinder et al. 2007, Eclesia et al. 2016). However, plant breeding has traditionally selected for aboveground yields alone; therefore, potential trade-offs between yield and root production must be carefully evaluated (DeHaan et al. 2005). New tools for monitoring root systems and in situ SOM in the ﬁeld are needed (Molon et al. 2017) [Jackson 2017: 422]. . . .
The importance of root inputs for SOM formation is likely attributable to both their chemical composition and, almost certainly, their presence in the soil; upon death, they immediately interact with soil minerals, microbes, and aggregates. Roots tend to be characterized more by aliphatic compounds that are readily sorbed to mineral surfaces, and their composition (and that of root exudates) can increase microbial carbon use efﬁciency (CUE), deﬁned as the ratio of microbial growth to carbon uptake, more than litter can. High CUE promotes microbial growth and carbon stabilization in mineral-associated soil pools, and low CUE favors biomass respiration (Manzonietal.2012a) [Jackson 2017: 423]. . . .
Soils hold the largest biogeochemically active terrestrial carbon pool on Earth and are critical for stabilizing atmospheric CO2 concentrations. Nonetheless, global pressures on soils continue from changes in land management, including the need for increasing bioenergy and food production [Jackson 2017: 420].
. . . plant breeding has traditionally selected for aboveground yields alone; therefore, potential trade-offs between yield and root production must be carefully evaluated [Jackson 2017].
Jackson, Robert B., et al, 2017, The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls, Annual Review of Ecology, Evolution, and Systematics 48:419–45, https://www.annualreviews.org/doi/abs/10.1146/annurev-ecolsys-112414-054234.