Loss of deep roots limits biogenic agents of soil development that are only partially restored by decades of forest regeneration
Abstract
© 2018 The Author(s). Roots and associated microbes generate acid-forming CO2 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.
Type
Journal articleSubject
Science & TechnologyLife Sciences & Biomedicine
Physical Sciences
Environmental Sciences
Meteorology & Atmospheric Sciences
Environmental Sciences & Ecology
Biotic weathering
critical zone biogeochemistry
land conversion
soil organic acids
in situ CO2
forest succession
ORGANIC-CARBON DYNAMICS
MICROBIAL COMMUNITY
VERTICAL-DISTRIBUTION
LAND-USE
DEPTH
MATTER
ACCUMULATION
BACTERIAL
PROFILES
ZONE
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https://hdl.handle.net/10161/21234Published Version (Please cite this version)
10.1525/elementa.287Publication Info
Billings, SA; Hirmas, D; Sullivan, PL; Lehmeier, CA; Bagchi, S; Min, K; ... De Richter,
DB (2018). Loss of deep roots limits biogenic agents of soil development that are only partially
restored by decades of forest regeneration. Elementa, 6(1). pp. 34-34. 10.1525/elementa.287. Retrieved from https://hdl.handle.net/10161/21234.This is constructed from limited available data and may be imprecise. To cite this
article, please review & use the official citation provided by the journal.
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Zach Brecheisen
Associate In Research

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