Soil organic matter formation and loss are mediated by root exudates in a temperate forest

The amount and composition of root exudates-low-molecular-weight carbon compounds released from living plant roots into soil-are expected to shift under global change, and a growing body of work indicates that root exudates have important impacts on stable soil organic matter dynamics. However, most research on exudate effects on soil organic matter uses highly homogenized or artificial soil, leaving major uncertainties in how exudates will influence carbon dynamics in natural, intact soil systems. We used C-13-labelled artificial root exudates to examine the effects of exudation rate and type on stable soil organic matter formation and loss in intact forest soil cores collected over a heterogeneous gradient in a temperate hardwood forest. We observed effects of different exudate treatments on stable soil carbon dynamics that overrode native soil heterogeneity, and higher exudation rates enhanced mineral-associated organic matter turnover but not accumulation. Organic and amino acid exudates led to net mineral-associated organic matter accumulation, with amino acids having particularly strong positive effects on microbial biomass. Simple sugars increased mineral-associated organic matter turnover (both formation and loss) but did not alter the size of this pool. Our results suggest that predicted increases in root exudation rates and compositional shifts towards simple sugars under global change may reduce soils' C storage capacity. The amount and composition of carbon compounds released from plant roots into soil influences soil carbon formation and loss, according to an artificial root exudate experiment using intact soil cores from a temperate forest.

Automated summary

The exudation rate and type of root exudates have significant impacts on soil carbon dynamics, particularly on the formation and loss of stable soil organic matter. Different exudate treatments affect the turnover of mineral-associated organic matter, with higher exudation rates enhancing turnover and simple sugars promoting both formation and loss.