On Earth’s surface, countless plants and animals grow and move, taking in and releasing carbon. Beneath all that, the soil teems with insects, microbes, and fungi, all bustling and respiring. The bedrock farther below the surface, in contrast, may seem sterile, although it, too, often hosts life. Trees and other plants commonly work their roots all the way down into bedrock fissures, bringing along accompanying microbes. This deep rooting markedly changes the chemistry of that fractured bedrock and the water that flows through it.
In a new study, Tune et al. investigated carbon cycling and fluxes within weathered bedrock several meters beneath an old-growth forest in Northern California. They note patterns in carbon dynamics that followed seasonal cycles. In the wet season, up to about 80% of the carbon produced in the bedrock dissolved into water and accelerated further weathering in the rock and soil. In the dry season, bedrock respiration continued as trees drew water from deeper bedrock zones when the soil above dried out.
The researchers also note that dissolved inorganic carbon (DIC) flowed downslope at their hillside study site and accumulated in the groundwater at the hill’s base and that DIC originating from the weathered bedrock accounted for 63%–80% of the total amount exported from the hillslope. This process thus carries the effects of bedrock weathering on the slope beyond the immediate ecosystem.
With this study, the researchers contribute to the study of an underresearched carbon source that has yet to be adequately factored into scientists’ understanding of the global carbon cycle. Future research, they suggest, should expand on these findings to look at conditions in other seasons, ecosystems, and scales to illuminate the role of weathered bedrock and deep rooting in carbon cycling and climate dynamics. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG005795, 2020)
—Elizabeth Thompson, Science Writer