Root ‘collaboration’ dimension of the root economics space drives variations in soil organic carbon among forest plantations by regulating particulate organic carbon

Growing evidence shows that plant roots play a more important role than aboveground organs in soil organic carbon (SOC) storage and stabilization. The root economics space defines root resource acquisition strategies along two axes: the ‘conservation’ and ‘collaboration’ dimensions, which are widely used to investigate the relationships between root traits and ecosystem functions. However, it remains unclear whether and how the two-dimensional root resource acquisition strategies of trees are linked to the concentrations of SOC and its fractions (i.e., particulate organic carbon, POC, and mineral-associated organic carbon, MAOC). In this study, we measured four core traits of the root economics space, absorptive root biomass, SOC, POC and MAOC concentrations, soil microbial necromass carbon (MNC) concentrations and two soil C-degrading enzyme activities in a 19-year common garden of six temperate tree species in northeastern China. We found that the root ‘conservation’ dimension had limited impacts on POC, MAOC or SOC concentrations. In comparison, the root ‘collaboration’ dimension was positively correlated with POC and SOC concentrations but not associated with MAOC concentrations. That is, angiosperm tree species with the ‘do-it-yourself’ strategy along the root ‘collaboration’ dimension had higher POC concentrations than gymnosperm tree species with the ‘outsourcing’ strategy, thus increasing SOC concentrations. Moreover, the root resource acquisition strategies did not affect SOC by regulating MNC, except that the root ‘collaboration’ dimension was positively correlated with bacterial necromass carbon (BNC) concentrations. Overall, our results demonstrate that root ‘collaboration’ rather than ‘conservation’ dimension of the root economics space drives forest SOC sequestration by regulating POC, which highlights that symbiotic fungi play an important role in regulating the relationships between plant ecological strategies and soil functions. Our findings provide a basis for tree species selection in afforestation to enhance SOC storage via root traits and for incorporating root traits into global forest SOC cycling models.