Biogeochemical Cycling and Mineral Weathering in Soil.

Weathering of minerals through biogeochemical processes is an important component of soil formation and plays a critical role in providing nutrients to the soil community, including plants, fungi, and bacteria. On longer time scales mineral weathering may have a significant impact on atmospheric carbon dioxide through the mechanisms of the long-term carbon cycle. Some plants have developed characteristics such as symbiotic relationships with fungi, deeper roots, and exudation of organic molecules, but it is poorly understood what impact these traits may have on mineral weathering. We hypothesize that different evolutionary groups of plants (e.g. angiosperms and gymnosperms) will have measurably distinct impacts on soil mineral weathering and related biogeochemical cycles.

We observed differences in mineral weathering and soil chemical profiles between the studied angiosperm and gymnosperm tree systems in the field, which include understory vegetation, fungi, and bacteria. We assessed rooting depth and density, tree biomass, leaf chemical composition, and element accumulation and depletion patterns in soil beneath the different tree types. Complementary laboratory studies illuminated differences in mineral surface etching, organic acid exudation, litter decomposition, and rhizosphere pH. Using these data, we have developed a simple model that illustrates the biogeochemical cycles relating to the study trees. A deeper understanding of the differences in mineral weathering and biogeochemical cycling of elements will help us anticipate the impact of shifting vegetation and soil communities on modern soils as well as potential implications for the role of plants, fungi, and bacteria in the global long-term carbon cycle.

These studies provide a snapshot of the biogeochemical factors at work in soil processes, but additional methods are needed to determine the dynamics and feedback loops at play within the soil. One potential method, a colorimetric functional assay for small organic molecules with strong metal-chelating characteristics known as siderophores, will be presented.