Integration of geochemical mass balance with sediment transport to calculate rates of soil chemical weathering and transport on hillslopes

Abstract

We developed a process‐oriented hillslope soil mass balance model that integrates chemical and physical processes within hillslope soils. The model explicitly factors that soil chemical weathering at any hillslope position is related to the flux of soil eroded from upslope as well as soil production from underlying bedrock. The model was merged with measurements of soil elemental chemistry and cosmogenic radionuclide‐based saprolite‐to‐soil conversion rates along a 50 m transect of a semiarid granodiorite hillslope in the southeastern Australian highlands. Inverse modeling results in the simultaneous quantification of the rates of soil chemical weathering and soil transport as a function of hillslope position. Soil chemical weathering rates per land surface area systematically varied along the transect from losses of 0.035 kg m−2 yr−1 on the ridge to gains of 0.035 kg m−2 yr−1 at the lowest slope position. The mass loss via soil chemical weathering would have been overestimated by 40% if the impact of soil transport on soil chemistry was ignored. The chemical mobility of elements, combined with biological nutrient demand, controlled the spatial redistribution of individual elements: P and Ca were preferentially retained relative to Si, Al, and Fe within the hillslope base. The calculated soil transport rate is linearly related to the product of soil thickness and slope gradient, instead of slope alone. Soil residence time was determined by calculating the time length for a 3 dimensional box (volume = 1 m2 surface area × soil thickness) to be entirely removed by mass flux of soil transport: 4 ka on the ridge to 0.9 ka at the hillslope base. These soil residence times, combined with soil chemical weathering rates, indicate that a 1 m2 area of soil loses ∼1,800 kg via chemical weathering while passing through the upslope portion of the hillslope, but that it regains ∼90 kg, probably via clay precipitation and biological retention of cations, during its passage through the lower segments of the transect. This study provides a previously unrecognized linkage between physical soil transport and soil chemical weathering that have implications for hillslope evolution as well as biogeochemistry.