Document: JUL-3-6-7

Soil aggregation in the rhizosphere: optimal conditions for multiple mechanisms.

JASTROW, J.D.* and R.M.MILLER

Argonne National Laboratory, Argonne, IL 60439, USA ¹

Abstract:
Soil aggregate structure together with its influence on porosity is a major controlling factor for virtually all soil processes. Moreover, the physical protection of organic debris within stable aggregates is an important mechanism enabling the accumulation of soil organic matter and nutrient reserves. In soils with a legacy of long-term exploration by roots, an aggregate hierarchy often develops, where primary particles and clay microstructures are bound into increasingly larger aggregates. Nearly ideal conditions for aggregate formation and stabilization exist in the rhizosphere. 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 root exudates and decomposition products are converted into polysaccharides and humic substances by a diverse microbial and faunal community. Roots and the hyphae of mycorrhizal fungi enmesh primary particles and microaggregates to form macroaggregates. The deposition of plant and microbially derived compounds and of glomalin (a glycoproteinaceous substance exuded by mycorrhizal hyphae) cements soil particles, organic debris, and smaller aggregates together to stabilize larger aggregates. As roots and hyphae die, particulate organic matter is deposited throughout the aggregate structure of the soil, where its decomposition stabilizes the pores within both macroaggregates and microaggregates. In a chronosequence of tallgrass prairie restorations, we observed that many binding agents are correlated with the formation of stable soil aggregates. However, by using path analysis to investigate the causal interrelationships of these agents, we find that the direct and indirect effects of roots dominate the effects of other binding mechanisms and thereby demonstrate the overall importance of rhizospheric processes to aggregation in grassland soils.

Keywords: rhizosphere, soil aggregation, roots, mycorrhizal fungi, soil organic matter

This abstract is being presented at: 9:45 AM in session:
Symposium # 21: The Rhizosphere: Top-Down and Bottom-Up Approaches.