Contributions of mycorrhizae to the development of soil aggregate hierarchy

Invited Speaker
MONDAY AUGUST 5: 10:30-12:00
"Microbial Interactions in the Mycorrhizosphere"
Organizers: Mike Miller (USA), John Garbaye (France)

JASTROW, J.D.
Environmental Research Division, Argonne National Laboratory, Argonne, IL 60439, USA.
Abstract
In soils where organic matter is the major binding agent, the formation of aggregates appears to be hierarchical in that primary particles and clay microstructures are bound into microaggregates (up to 250 µm in diameter), which, in turn, are bound into macroaggregates (up to several millimeters in diameter). Mycorrhizae play several important roles in the aggregation process at more than one hierarchical level. Roots and the hyphae of mycorrhizal fungi are believed to function as a ³sticky string bag,² enmeshing primary particles and microaggregates together to form macroaggregates and enabling further stabilization to occur as biological activity within the macroaggregate results in the deposition of plant and microbially derived polysaccharides and mucigels. Using a path analysis approach with data from a chronosequence of prairie restorations, we found the relative contributions of roots and external hyphae to macroaggregate formation were greater than the contributions of microbial biomass, hot-water soluble carbohydrates, and soil organic carbon, especially for larger macroaggregates. Other studies of soils from the same site have identified a glycoproteinaceous substance exuded by mycorrhizal hyphae that appears to cement soil particles and smaller aggregates together and may be a major contributor to the ³sticky² mechanism of aggregate stabilization. In addition, the hydrophobic nature of this substance may indirectly contribute to aggregate stability by dampening the disruptive forces of rapid water movement into the pores between and within aggregates. At the microaggregate scale, evidence that partially decomposed hyphal fragments encrusted with clays can serve as nucleating sites for microaggregates has been obtained from soil micromorphology studies.