Loss of deep roots limits biogenic agents of soil development that are only partially restored by decades of forest regeneration

Roots and associated microbes generate acid-forming CO 2 and organic acids and accelerate mineral weathering deep within Earth’s critical zone (CZ). At the Calhoun CZ Observatory in the USA’s Southern Piedmont, we tested the hypothesis that deforestation-induced deep root losses reduce root- and microbially-mediated weathering agents well below maximum root density (to 5 m), and impart land-use legacies even after ~70 y of forest regeneration. In forested plots, root density declined with depth to 200 cm; in cultivated plots, roots approached zero at depths >70 cm. Below 70 cm, root densities in old-growth forests averaged 2.1 times those in regenerating forests. Modeled root distributions suggest declines in density with depth were steepest in agricultural plots, and least severe in old-growth forests. Root densities influenced biogeochemical environments in multiple ways. Microbial community composition varied with land use from surface horizons to 500 cm; relative abundance of root-associated bacteria was greater in old-growth soils than in regenerating forests, particularly at 100–150 cm. At 500 cm in old-growth forests, salt-extractable organic C (EOC), an organic acid proxy, was 8.8 and 12.5 times that in regenerating forest and agricultural soils, respectively. The proportion of soil organic carbon comprised of EOC was greater in old-growth forests (20.0 ± 2.6%) compared to regenerating forests (2.1 ± 1.1) and agricultural soils (1.9 ± 0.9%). Between 20 and 500 cm, [EOC] increased more with root density in old-growth relative to regenerating forests. At 300 cm, in situ growing season [CO2] was significantly greater in old-growth forests relative to regenerating forests and cultivated plots; at 300 and 500 cm, cultivated soil [CO2] was significantly lower than in forests. Microbially-respired δ13C-CO2 suggests that microbes may rely partially on crop residue even after ~70 y of forest regeneration. We assert that forest conversion to frequently disturbed ecosystems limits deep roots and reduces biotic generation of downward-propagating weathering agents.