ROOT AND MYCORRHIZA IMPACTS ON DEEP SOIL: SOIL PRESERVATION OF RHIZOSPHERE PROCESSES

The bioturbated A-horizon is continually subject to rhizosphere processes of growing roots. When roots senesce the root and its rhizosphere are mixed back into the bulk soil, through bioturbation and decomposition processes. Below the zone of bioturbation there are fewer roots, but when senesced roots decay the physical and geochemical changes induced by root processes remain, marking the passage of the root through the deep soil. Rhizosphere channels below the bioturbated zone remain zones of higher carbon content, secondary mineral formation and weathered primary minerals.

We used soils in the Santa Cruz marine terrace chronosequence to explore deep rhizosphere processes. The chronosequence displays a sequence of reticulate mottling from the youngest soil (65 ka) without mottles to the oldest soil (225 ka) with well-developed mottles. Mottles develop in soils formed from relatively uniform shoreline sediments, below the depth of bioturbation. They consist of an interconnected network of clay and carbon enriched zones (gray) bordered by bleached parent material (white) within a diminishing matrix of oxidized parent material (orange). To explore the nature of mottle development, physical and chemical characteristics of mottle separates (orange, gray, and white) were compared. Mineralogical, isotopic, and surface-area differences among mottle separates indicate that centimeter-scale mass-transfer acting across millennia is an integral part of weathering-front propagation, pedogenesis, and carbon and nutrient transfer. Elemental analysis, electron microscopy, and iron-isotope systematics indicate that mottle development is driven by deep roots together with their fungal and microbial symbionts. Thus, we conclude deep soil horizons on old stable landforms develop reticulate mottling as the long-term imprint of rhizospheric processes controlling pedogenesis, plant-community sustenance, and sequestration of carbon at depth in unsaturated zones.