Soils and Carbon Transformations in Northern Michigan Forested Watersheds

By controlling pH and redox chemistry, organic/inorganic carbon transformations affect important soil processes such as mineral weathering and the transport of trace metals. Carbon cyling was studied using two natural forested sites (Cheboygan and Tahquamenon watersheds) and using results from chambered tree-growth experiments conducted under different treatments of soil fertility and PCO₂. Although both watersheds are established on sandy glacial drift deposits, Cheboygan soils are carbonate-rich, while Tahquamenon soils are carbonate-poor. The occurrence and transformation of organic and inorganic carbon was followed in soils and soil solutions from different forest types: aspen and mixed hardwood forests in the Cheboygan watershed, and conifer and mixed hardwood forests in the Tahquamenon watershed. Geochemical characterization of soils, soil solutions, shallow groundwaters, and streams included major, minor, and trace elements in addition to inorganic and organic carbon concentrations. Soil and soil water chemistries were also followed over the 2 years of chambered tree growth experiments. Tahquamenon soils lack inorganic carbon to at least a depth of 150 cm, while inorganic carbon is present in Cheboygan soils at depths greater than 80 cm. Tahquamenon soils have higher organic carbon contents than Cheboygan soils. Shallow groundwaters and stream waters of both Tahquamenon and Cheboygan watersheds are Ca-Mg-HCO₃ solutions, but Cheboygan waters are up to twice as concentrated, due to the abundance of reactive carbonate minerals in the soil, higher respiration rates, and higher growing season soil gas CO₂ values. Carbon in both watersheds is present in shallow soil solutions as DOC derived from organic matter decomposition, and is rapidly transformed and replaced by DIC from organic matter oxidation, respiration, and carbonate dissolution with increasing depth in the soil column. The DOC to DIC transition occurs at different depths in different forest soils and is related to the underlying geology and vegetation, both of which influence mineral weathering and soil development. In the two watersheds, a positive correlation exists between DOC and metals such as Pb and Al, and soil solutions with low pH and high DOC contain higher trace metal concentrations than stream waters. Similarly, weathering of silicate minerals is enhanced at shallow depths by organic ligands present in DOC, and carbonate mineral weathering occurs deeper in the soils. While DOC and DIC maxima generally occur at different soil column depths in natural forests, our work in the tree-growth chambers shows that in disturbed soils, mineral weathering processes facilitated by DOC and DIC may occur together at shallow soil depths. Homogenization of the chamber soils brought reactive carbonate minerals up to shallow depths into the zone of highest PCO₂, allowing for dissolution of carbonate minerals to occur simultaneously with silicate mineral dissolution. Soil disturbance due to development or agriculture in carbonate-rich areas may thereby increase DIC fluxes from the landscape while decreasing transport of trace metals by moderating pH.