Online

Bower, Jenny

Department of Plant and Soil Science

2023

Ph. D.

Soil formation has long been framed as a vertical process because of the historical emphasis on agricultural areas, which tend to be low relief. This framing fails in areas where steep topography, a restrictive soil horizon, or shallow bedrock leads to lateral flow. In the glaciated Northeast, lateral podzolization is the dominant process forming shallow-bedrock soils on hillslope summits and shoulders, in contrast to deeper soils on backslopes that are influenced by vertical podzolization. These patterns are accompanied by gradients in hydrologic regime and solution chemistry which imply the existence of mineral weathering gradients. This work aimed to understand weathering dynamics across hillslopes affected by lateral and vertical podzolization. Whereas the fine fraction ([less than or equal to] 2 mm) is traditionally the focus of soil studies, these potential weathering gradients were hypothesized to affect all mineral sources. Therefore, a whole-regolith approach was employed to holistically investigate weathering in the fine fraction, rock fragments, and bedrock. Sampling was conducted within the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, at three locations in Watershed 3, the hydrologic reference watershed. Podzols were described and characterized by genetic horizon. Soil samples, rock fragments, and rock cores were collected from 12 closely spaced sites along 3 hillslope transects. Electron microprobe analysis and mass transfer coefficients were employed independently to estimate the depletion of plagioclase and plagioclase-bound elements in the fine fraction of laterally and vertically formed podzols. X-ray spectrometry and electron microprobe analysis were used to estimate porosity in rock fragments and compare mineralogy between rock fragments and the surrounding fine fraction. The distribution of weathering-derived metals was investigated through elemental analysis and wet-chemical extractions. Plagioclase losses were found to be higher in lateral eluvial podzols compared to vertical podzols, a difference that persisted when comparing the same genetic horizons in soils at different hillslope positions. The outer edge of rock fragments in lateral eluvial soils had higher mean porosity than rock fragments in vertical soils. Porosity in rock fragments increased as a power of the fractional mass loss of sodium estimated within the fine fraction of soils. Rocks exhibited weathering-related trends in abundance, lithology, and secondary metals down the hillslope. Rocks were the principal source of extracted pedogenic metals in areas with bedrock outcrops and shallow eluvial soils. Notably, this contrasted with soils downslope, where the fine fraction was the main source of pedogenic metals. This work shows that weathering in the watershed is controlled by hillslope soil patterns, which in turn are formed by local gradients in flushing conditions and topography. Results highlight the contributions of coarse fractions and bedrock to soil weathering and demonstrate that weathering intensity is organized along spatial gradients in glaciated catchments. These findings are relevant to biogeochemical models, regional estimates of weathering fluxes, and calculations of critical loads used to evaluate ecosystem sensitivity to air pollution.