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Soil mixing plays a significant role in contaminant transport, carbon sequestration, and landscape evolution, yet the rates and driving mechanisms are poorly constrained. Here we use depth profiles and advection-diffusion modeling of fallout nuclides to quantify differences in short-term (<100 yr) physical soil mixing across contrasting landscapes. We constrain advection in soils using the distribution of cosmogenic 7Be and weapons-derived isotopes, and quantify mixing with a steady-state model of vertical 210Pb transport. On a forested landscape in the Bega Valley in southeastern Australia and on grasslands in Mann County, California, where bioturbation is documented as the dominant sediment transport mechanism, we calculate diffusion-like mixing coefficients of 1-2 cm² yr-1. In montane forest soils of northern New England, we observe little field evidence of short-term mixing, and find that the traditional advection-diffusion model fails to describe 210Pb profiles. Because nuclide profiles here can be described with a simple model of litterfall, organic matter decay, and radioactive decay, we argue that diffusion-like processes are barely active on short time scales, and that the advection-diffusion model overestimates diffusion-like transport. While animal bioturbation and soil freezing cycles have little effect on the fate of elements in New England, physical soil mixing drives transport at Bega Valley and Mann County. We suggest that the absence of soil stirring that we quantify in New England forests may explain the slow physical erosion here (~0.2 cm/k.y.) relative to the actively bioturbated soils of Bega Valley and Mann County (5-10 cm/k.y.). [ABSTRACT FROM AUTHOR]

The activity of fallout radionuclides (7Be, 137Cs, and 210Pb) was measured on upland and floodplain soils and on suspended sediments to quantify sources of fine sediment and to estimate sediment transport distances in stream channels in the Yellowstone River basin. Samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of Soda Butte Creek to Billings, Montana, a 423-km-long reach of channel. The inventory of radionuclides in soil increases with precipitation and is highest in the headwaters. The activity of radionuclides in suspended sediment decreases downstream, and more activity is observed earlier than later in the flood hydrograph. The radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion. Fine suspended sediment has an intermediate radionuclide signature that is quantified in terms of the relative contribution of these two sources of fine sediment. At sites high in the drainage, soils contribute 50% to the suspended load and this value decreases to 11%–26% downstream. Fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source. Transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream. These estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure. This study of a large watershed confirms earlier results from smaller basins and suggests that transport distances increase with basin size. [ABSTRACT FROM AUTHOR]