Physical vs. Chemical Weathering Controls of Soils’ Capacity to Store Carbon: Hillslope Transects under Different Climatic Conditions

Abstract:

Soil C storage is balanced by photosynthetic production and microbial decomposition of organic matter (OM). Recently, this view has been expanded to account for the effects of physical erosion of OM in determining soil C storage. In parallel, the focus on OM quality as a primary determinant of C turnover has shifted to OM-mineral interactions. These recent advances necessitates our ability to discern how physical erosion, which controls the production, breakdown, and removal of colluvial soils, and chemical weathering, which generates secondary phyllosilicate and iron oxides, independently and collaboratively affect soils’ capacity to store C. Here we present soil organic C contents and storages as a function of soil properties that are controlled by physical vs. chemical weathering processes. The study site includes two hillslopes under different climates in SW Australia. The wetter site has continuous canopy of eucalyptus, while the drier site is covered by grasses with scattered eucalyptus overstorey. The two hillslope transects share similar granodiorite parent materials and denudation rates. Bioturbation-driven soil creep appears equally effective at both sites. In eroding areas, chemical weathering has created greater mineral surface area in the soils of wetter site, while physical soil production and erosion resulted in forming the eroding soils of similar thicknesses at both sites. In the drier site, however, vegetation density varies significantly with topography-dependent soil moisture, which appears to have resulted in a soil toposequence where impacts of localized overland-flow erosion is evident through soil mineral surface area, texture, and C contents. These soil properties, in contrast, are largely homogeneous across the wetter hillslope transect presumably because of the lack of localized overland-flow erosion. As a result, at the depositional areas, the drier site exhibits greater or similar soil C storages, which sharply contrasts with the smaller amount of C stored in the eroding soils at the drier site. These results reveal that climate affects relative significance of physical vs. chemical weathering processes in hillslopes and that such differential intensities of the two mechanisms may significantly interfere with biologically mediated climate control of soil C storages.