Quantification of the effects of angiosperms and gymnosperms on silicate weathering and related soil nutrient cycling: Implications for phanerozoic atmospheric carbon dioxide and modern soil trace metal pollution

Accelerated weathering of terrestrial silicate minerals by vascular plants is an important process in the long-term carbon cycle and its effect on atmospheric carbon dioxide levels over geological time. The ancestors of modern gymnosperms evolved in the Paleozoic and lowered paleo-atmosphere CO2 through increased continental weathering. It is unclear what additional influence the evolution of the angiosperms during the Mesozoic may have contributed. To address the impact of angiosperm evolution on past weathering is necessary to first understand the relative weathering rates of the modern trees.;To test whether angiosperms or gymnosperms accelerate silicate weathering more, I conducted laboratory experiments in parallel with a field study in the Cascade Mountains, WA. The experiments explored the weathering of basalt and granite minerals by tree seedlings and mature litter. Gymnosperm seedlings and litter both produce lower soil pH and higher concentrations of chelating organic acids as compared to the angiosperms. Mineral weathering is similar beneath both types of litter, but the gymnosperm seedlings promote greater silicate dissolution per unit root biomass than angiosperm seedlings.;In the field, adjacent stands of deciduous angiosperms and evergreen gymnosperms provided a natural laboratory to study the net release of major and trace elements in shallow granodiorite soils. The combination of the differences between angiosperm and gymnosperm mineral weathering, vegetation chemistry, and root density distribution produced the greatest weathering in the top few centimeters of soil under the gymnosperms. The accumulation of sonic metals beneath the angiosperms has implications for both modern and geologic processes. Trace metal accumulation and mobilization under the two tree types may be important for mediating or exacerbating heavy metal toxicity in polluted soils. Magnesium and calcium are important cations for the geological carbon cycle. Magnesium is retained in the angiosperm soil system due to lower rates of hornblende etching and greater recycling of magnesium in deciduous leaf litter. Calcium is released in similar amounts from beneath both angiosperms and gymnosperms. Therefore, this reduction in magnesium flux from the soils suggests that the rise of the angiosperms may have a muted influence on atmospheric CO 2 via silicate weathering if weathering in shallow soils is dominant.