How do roots restructure water and carbon dynamics in the critical zone?

Roots are physical and chemical engineers of the subsurface that are sensitive to changes in climate, and whose power to reshape the subsurface differs with land cover. Roots create and destroy porosity through enmeshment of particles, lateral and vertical boring through regolith, and cleaving of rocks from parent material. Their ability to translocate water, exude sugars and acids, and take up solutes influences hydrologic connectivity, water residence times, carbon transport and transformation, microbial access to resources, and chemical equilibrium conditions. Analysis of land-cover datasets suggest that root depth distributions are changing globally, shallowing in agricultural environments and deepening with woody encroachment. Yet where, when, and how changes in root distributions alter water and carbon dynamics in the critical zone is not well known. Using data generated at environmental observatories across the U.S. Long-Term Ecological Research program, the Critical Zone Collaborative Network, National Ecological Observatory Network, and the Department of Energy Watershed Focus Areas in combination with the Pedogenic and Environmental Dataset (PEDS), we ask: How do roots shape regolith hydrology and carbon dynamics? 

A clear signal is emerging from grassland, forest, and agricultural sites across the U.S. that indicates changes in rooting dynamics have measurable and meaningful impacts on critical zone functions. Evidence shows that changes from forest to crop and back to forest impacts soil structure deep beneath the plow line in systematic ways. Losses of rooting abundance upon conversion of grasslands to agriculture affects the propensity of organic carbon to form and protect aggregates throughout the subsoil. Reduced fire frequency at tallgrass prairie sites in the Midwest have led to rapid woody expansion in recent decades. Where woody encroachment persists, coarse roots, smaller mean soil aggregate diameters, and more readily destabilized carbon pools proliferate. Encroachment of woody plants increases the infiltration of soil water rich in  CO₂ into deep rocks and enhances carbonate weathering as predicted by models. These woody plants rely on deeper water sources, draw soil moisture down to a greater degree at depth, and are likely responsible for reducing streamflow and changing the timing of groundwater contributions to the stream. At Rocky Mountains sites dominated by conifers and aspen, coarse- and fine-root abundances are elevated under aspen in the upper 75 cm of the soil profile compared to conifer sites. Elevated soil organic carbon, lower extractable organic carbon, lower C:N values and elevated enzyme activity indicate soil carbon under aspen is likely more stable as a result of more microbial processing. Finally, in a predominantly Douglas-fir forest in the Pacific Northwest, second-growth forests exhibit substantially fewer fine roots at depths <50 cm, which appears to exert control on nitrogen availability in this nutrient-limited system and thus potentially limits carbon stability as more extractable organic carbon is generated from second-growth forests at depth. Data from these sites demonstrate how alterations to rooting distributions change the physical structure and moisture status of soil, and may be linked to carbon stability as the proportion of fine and coarse roots dictate overall access to carbon pools.