Evidence for the primacy of living root inputs, not root or shoot litter, in forming soil organic carbon

Abstract

Soil organic carbon (SOC) is primarily formed from plant inputs, but the relative carbon (C) contributions from living root inputs (i.e. rhizodeposits) vs litter inputs (i.e. root + shoot litter) are poorly understood. Recent theory suggests that living root inputs exert a disproportionate influence on SOC formation, but few field studies have explicitly tested this by separately tracking living root vs litter inputs as they move through the soil food web and into distinct SOC pools. We used a manipulative field experiment with an annual C grass in a forest understory to differentially track its living root vs litter inputs into the soil and to assess net SOC formation over multiple years. We show that living root inputs are 2-13 times more efficient than litter inputs in forming both slow-cycling, mineral-associated SOC as well as fast-cycling, particulate organic C. Furthermore, we demonstrate that living root inputs are more efficiently anabolized by the soil microbial community en route to the mineral-associated SOC pool (dubbed 'the in vivo microbial turnover pathway'). Overall, our findings provide support for the primacy of living root inputs in forming SOC. However, we also highlight the possibility of nonadditive effects of living root and litter inputs, which may deplete SOC pools despite greater SOC formation rates.

Pathways of soil organic matter formation from above and belowground inputs in a Sorghum bicolor bioenergy crop

“…It is also consistent with the few studies that compared SOM formation from root and shoot residues. Contribution to SOM formation from roots is often found to be much higher than from leaves (Berhongaray, Cotrufo, Janssens, & Ceulemans, 2018;Sokol et al, 2018), but leads to higher POM or POM like fractions (Bird et al, 2008), while leaf litters often exhibit a relatively higher contribution to MAOM or persistent SOM fractions (Hatton, Castanha, Torn, & Bird, 2015;Lavallee et al, 2018). While supporting the idea that more SOM can be formed from root residues in annual crops where they often have a higher recalcitrance than above-ground residues, these findings suggest that the mean residence time of root residuesderived SOM is not higher than that formed from above-ground residue decomposition.…”

“…This conflicts with Rasse, Rumpel, and Dignac (2005), who found that the mean residence time of rootderived C in soils is 2.4 times greater than shoot-derived C, and Jackson et al (2017) as likely to be stabilized as SOM, perhaps higher in agricultural systems. Indeed, this study only investigated residues inputs, and root exudates given their inherent lability may efficiently form persistent SOM (Cotrufo et al, 2013;Sokol et al, 2018;Strickland, Wickings, & Bradford, 2012). We believe that the fractionation of SOM into physical fractions of known stabilization mechanisms can help predict the persistence of residue derived C and N in soil.…”

“…The ability of crop residues to contribute to SOM formation has traditionally focused on aboveground biomass, or shoots. However, live and dead roots are the major actors in the formation, stabilization, and destabilization of SOM (Sokol, Kuebbing, Karlsen-Ayala, & Bradford, 2018), and their contribution to SOM formation has started to receive significant attention (Balesdent & Balabane, 1996;Kong & Six, 2010;Lavallee, Conant, Paul, & Cotrufo, 2018;Rasse et al, 2006;Schmidt et al, 2011). Yet, we lack a complete understanding of how root vs shoot detritus form SOM into differently protected physical fractions, and contribute to soil C and N storage and/or recycling.…”

“…As an extension, biochemically recalcitrant inputs would preferentially accumulate as POM (Castellano, Mueller, Olk, Sawyer, & Six, 2015). While this hypothesis has been tested and often confirmed in the laboratory (Córdova et al, 2018;Haddix et al, 2016;Lavallee et al, 2018) field experiments are still limited and largely performed in natural forest or grassland systems (Bird, Kleber, & Torn, 2008;Bird & Torn, 2006;Sokol et al, 2018;Soong et al, 2016). Understanding how differences in litter chemistry impact the decomposition and resultant SOM formation of root vs shoots is going to be critical to informing agricultural management, particularly in bioenergy production systems.…”

Plant root exudation under drought: implications for ecosystem functioning

“…Interest in root exudation has spiralled with the realization of its importance in plant nutrient uptake (Mommer et al , ) and the formation of stable soil organic C (Sokol et al , ). In addition, root exudates form a direct communication pathway between plants and rhizosphere microbes and have the potential to influence plant tolerance and survival during severe abiotic stress.…”