EFFECTS OF ROOT AND RHIZOSPHERE PROCESSES ON DEEP SOILS AND BEDROCK

The A-horizon of soil is continually subjected to rhizospheric processes of growing roots. As 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; however when these deep roots senesce and decay the physical and biogeochemical changes induced by root processes remain, marking the passage of the root through the deep soil. Rhizospheric zones below bioturbated soil persist as regions of higher carbon content, secondary mineral formation, and weathered primary minerals.

SOM accumulation and stabilization over time is an important process as soils are a large carbon reservoir. The association of SOM with poorly crystalline or short-range-ordered secondary minerals is important for carbon stabilization. Fate of root exudates in deep soil and processes controlling the extent and spatial association of exudates with mineral phases are as yet undefined. We employ a variety of scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) techniques to image rhizosphere and associated carbon-mineral interactions. Sub-micron exploration of field relations provides valuable information on SOM-mineral interactions.

Soils of the Santa Cruz marine terrace chronosequence are used to illustrate changes in deep rhizosphere (> 1m) through time. Cracks and soil ped faces are often covered by a network of roots embedded in organic deposits (biofilm). In these soils microscopy reveals secondary clay minerals associated with and perhaps forming in organic-rich biofilms that occur along rooting zones (rhizosphere). We hypothesize that the synthesis of secondary clays in the rhizosphere is a mode of C incorporation in secondary minerals.