Soil stabilization of root-derived organic matter: Aggregate formation and physical protection of root inputs

Background/Question/Methods Nearly ideal conditions for soil aggregate formation and stabilization exist in the rhizosphere. In soils with a legacy of long-term exploration by roots, a hierarchical aggregate structure often develops. As fibrous roots grow, they exert pressures and locally dry the soil causing soil particles to be pushed and drawn together at the same time that exudates and rhizodeposits support a diverse microbial and faunal community. Roots and the hyphae of associated mycorrhizal fungi serve as a flexible latticework that enmeshes and stabilizes larger aggregates. Because root turnover often occurs within the inner space of soil aggregates, the decomposition process leads to the formation and stabilization of microaggregates within macroaggregates and development of an aggregate hierarchy. The resulting physical structure feeds back to impact decomposer access to substrates, air, water, and nutrients, thereby affecting decomposition and soil carbon cycling and sequestration. The objective of this talk is to synthesize results from a series of studies examining various aspects of this process via the gradients provided by a chronosequence of tallgrass prairie restorations, where frequent burning limits the inputs of surface litter to soil organic matter. Methods include soil physical fractionation, structural equation modeling, ultra-small angle x-ray scattering (USAXS), and microtomography. Results/Conclusions Early studies investigated the direct and indirect causal effects of roots and mycorrhizal hyphae on macroaggregate stabilization and demonstrated the spatial scale at which these effects operate. More recent research has confirmed this rhizosphere-dominated system has steadily accrued soil carbon for 30 years and explored the nature and spatial location of accumulated carbon. Most of the carbon accrual occurred in the silt-sized component of microaggregates stabilized within macroaggregates. However, rates of carbon accrual varied significantly among various physically isolated fractions, suggesting that some pools reach steady state faster than others or that the storage capacity of some pools appears to have saturated while other pools continue to accumulate carbon. USAXS studies indicate that a key mechanism limiting storage capacity is the change in pore architecture and pore filling that occurs via mineral encapsulation of colloidal organic matter. Tomographic evidence suggests continued carbon accrual then depends on progressive filling of larger scale pores. In conclusion, the stabilization of root-derived organic matter is facilitated by turnover of roots in intimate association with the self-organizing structure of soil solids and pores.