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Global sedimentation rates increased during the Quaternary due to frequent landscape adjustment to climatic oscillations. Although high rates of sediment transport are commonly associated with glacial conditions in mountainous terrain, the influence of climate and vegetation on geomorphic response is poorly constrained outside of glaciated settings. Along a low-gradient (<30%) hillslope-valley transect on a moderately dissected, loess-mantled fluvial terrace in the Charwell Basin, New Zealand, we coupled records of colluvial infilling and vegetation change (via phytoliths) to show that late Pleistocene sediment flux was ∼0.0012 m³ m-1 a-1 under a shrubland/grassland mosaic and Holocene sediment flux was κ0.0022 m³ m-1 a-1 under forest. This near doubling through the last glacial-interglacial transition appears to reflect increased bioturbation and downslope soil transport associated with a forest ecosystem. Such an increase in sediment transport contrasts with a contemporaneous decrease inferred for adjacent steep, landslide-prone catchments, suggesting that geomorphic response to climate change is heavily modulated by biology, topography, and geological substrate. Our findings appear to contradict the commonly cited notion that forest colonization universally stabilizes soils and suppresses erosion. Instead, over long time scales, bioturbation associated with forests may increase transport along gentle portions of the landscape not subject to slope instability or erosion by overland flow. [ABSTRACT FROM AUTHOR]

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]

Burrowing organisms assist in shaping earth surfaces and are simultaneously affected by the environment they inhabit; however, a conceptual framework is not yet available to describe this feedback. We introduce a model that connects the population density of soil-burrowing animals to sediment transport via energy. The model, combined with avail- able data from California hillslopes where soil erosion is driven by pocket gophers (Thomomys bottae), suggests that a gopher annually expends ∼9 kJ of energy, or ∼1% of reported burrowing energy expenditure, in generating sediment transport. The model is used to evaluate the case that gophers prefer to populate thicker soils. The results suggest that this behavior may drastically dampen the spatial and temporal variations of soil thickness and gopher populations, implying that burrowing organisms may create landscapes distinct from those affected by abiotic processes. [ABSTRACT FROM AUTHOR]