Rhizosphere priming is tightly associated with root-driven aggregate turnover

Abstract

The root-driven soil aggregate turnover dynamics and rhizosphere priming effect (RPE, changes in soil organic carbon (SOC) decomposition caused by living roots) are central to the understanding of SOC cycling. However, the association between aggregate turnover and the RPE has not been illuminated in plant-soil systems because of methodological difficulties. Using rare earth oxides to trace the transformations among different aggregates and 13C natural abundance labeling, we for the first time simultaneously investigated aggregate turnover and the RPE at two phenological stages of two grass species (Agropyron cristatum and Koeleria cristata): tillering (40 days after planting, DAP40) and jointing-heading (DAP63). We found that aggregate turnover rates varied widely, with a range between 0.006 day−1 and 0.024 day−1, i.e., turnover times (the reciprocal of turnover rates) ranged from 41 to 168 days, and were significantly influenced by plant species, sampling date and their interaction. Particularly, greater aggregate turnover rates (2% ~ 68%) and transformations in breakdown and formation pathways were found for K. cristata than for A. cristatum at DAP63. The RPEs increased with plant growth and ranged from −29% to +163%. Especially, the RPE and microbial biomass C were significantly greater for K. cristata than for A. cristatum at DAP63. Root-driven aggregate turnover was tightly associated with the RPE, possibly because of the release of aggregate-protected C for microbial decomposition. There was no net C loss mainly because increased aggregate formation could have sequestrated root-derived C in macroagrgegates and thus counteracted the C loss by the positive RPE. We therefore propose a new framework of root-driven aggregate turnover for considering how plant roots influence SOC dynamics via aggregate turnover. Root-accelerated aggregate turnover acts as a “key”: enhancing SOC decomposition (i.e. RPE), while simultaneously accelerating the occlusion of root-derived C and thus facilitating new C sequestration. This framework highlights that living root-driven aggregate turnover alters the physical protection of SOC and regulates the RPE, which aligns well with the emerging perspective of SOC stabilization.