WEATHERING OF ROCK TO REGOLITH: THE ACTIVITY OF DEEP ROOTS IN BEDROCK FRACTURES

It is generally assumed that rooting depth is restricted in shallow soils due to the underlying solid bedrock, and thus most studies of root dynamics focus on the uppermost soil horizons. However, in many landscapes, shallow soils overlie actively weathering bedrock. We tested the role of tree roots in weathering bedrock by excavating pits along a catena in a catchment with shale bedrock at the Susquehanna Shale Hills Critical Zone Observatory, PA. We measured a range of properties in the soil, rock, and rock fracture fill at a ridge top, mid-slope, toe-slope, and valley floor that include: 1) root density, distribution, and respiration, 2) soil gas, and 3) elemental compositions, mineralogy, and morphology. Root density decreased with depth, but fine roots were observed in rock fractures even in the deepest (~1.8 m), least weathered shale sampled. Root densities in fractures were similar in the upslope positions, but significantly lower (p < 0.01) in the toe-slope despite higher fracture density. Average root respiration (per dry root mass) in the soil and rock fractures was comparable. Thus, the total CO₂ flux from root respiration tracked root density, declining with depth. Microbial respiration, estimated with C mineralization potential, was ~10× lower than root respiration in both the soil and rock fractures. Roots were found only in >50 μm-aperture fractures filled with particulate material. Fracture fill was similar to the lowest soil horizons with respect to clay composition, element mobility, extractable dissolved organic C, inorganic N-species, and potentially mineralizable C and N, while total C and total N values were similar to the shale. In the bulk soil, depletion profiles (Al, Fe, K, Mg, and Si) relative to unweathered shale reflected weathering of illite and vermiculized chlorite to kaolinite and were similar between the soil and fracture fill. Such similarities indicate that the fracture fill is likely the result of pedogenic processes at depth rather than translocation of soil particles downward. Our data suggest that roots and fill in rock fractures down to ~1.8 m are qualitatively similar to those in surface soil horizons. Thus, the deepest manifestation of the chemical depletion profiles we observed consists of rock fracture fill, and this fill is present at low amounts with similarly low amounts of roots.