Browsing by Subject "LONG-TERM"

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Open Access
Boom and bust carbon-nitrogen dynamics during reforestation
(Ecological Modelling, 2017-09-24) Parolari, AJ; Mobley, ML; Bacon, AR; Katul, GG; Richter, DDB; Porporato, A
© 2017 Elsevier B.V. Legacies of historical land use strongly shape contemporary ecosystem dynamics. In old-field secondary forests, tree growth embodies a legacy of soil changes affected by previous cultivation. Three patterns of biomass accumulation during reforestation have been hypothesized previously, including monotonic to steady state, non-monotonic with a single peak then decay to steady state, and multiple oscillations around the steady state. In this paper, the conditions leading to the emergence of these patterns is analyzed. Using observations and models, we demonstrate that divergent reforestation patterns can be explained by contrasting time-scales in ecosystem carbon-nitrogen cycles that are influenced by land use legacies. Model analyses characterize non-monotonic plant-soil trajectories as either single peaks or multiple oscillations during an initial transient phase controlled by soil carbon-nitrogen conditions at the time of planting. Oscillations in plant and soil pools appear in modeled systems with rapid tree growth and low initial soil nitrogen, which stimulate nitrogen competition between trees and decomposers and lead the forest into a state of acute nitrogen deficiency. High initial soil nitrogen dampens oscillations, but enhances the magnitude of the tree biomass peak. These model results are supported by data derived from the long-running Calhoun Long-Term Soil-Ecosystem Experiment from 1957 to 2007. Observed carbon and nitrogen pools reveal distinct tree growth and decay phases, coincident with soil nitrogen depletion and partial re-accumulation. Further, contemporary tree biomass loss decreases with the legacy soil C:N ratio. These results support the idea that non-monotonic reforestation trajectories may result from initial transients in the plant-soil system affected by initial conditions derived from soil changes associated with land-use history.
Open Access
Mercury Sourcing and Sequestration in Weathering Profiles at Six Critical Zone Observatories
(Global Biogeochemical Cycles, 2018-10-01) Richardson, JB; Aguirre, AA; Buss, HL; Toby O'Geen, A; Gu, X; Rempe, DM; Richter, DDB
©2018. American Geophysical Union. All Rights Reserved. Mercury sequestration in regolith (soils + weathered bedrock) is an important ecosystem service of the critical zone. This has largely remained unexplored, due to the difficulty of sample collection and the assumption that Hg is predominantly sequestered within surface soils (here we define as 0–0.3 m). We measured Hg concentrations and inventories in weathering profiles at six Critical Zone Observatories (CZOs): Boulder Creek in the Front Range of Colorado, Calhoun in the South Carolina Piedmont, Eel River in coastal northern California, Luquillo in the tropical montane forest of Puerto Rico, Shale Hills of the valley and ridges of central Pennsylvania, and Southern Sierra in the Sierra Nevada range of California. Surface soils had higher Hg concentrations than the deepest regolith samples, except for Eel River, which had lower Hg concentrations in surface soils compared to regolith. Using Ti normalization, CZOs with <12% rock-derived Hg (Boulder Creek, Calhoun, and Southern Sierra) had Hg peaks between 1.5 and 8.0 m in depth. At CZOs with >50% rock-derived Hg, Eel River Hg concentrations and pools were greatest at >4.0 m in the weathering profile, while Luquillo and Shale Hills had peaks at the surface that diminished within 1.0 m of the surface. Hg and total organic C were only significantly correlated in regolith at Luquillo and Shale Hills CZOs, suggesting that Hg sorption to organic matter may be less dominant than clays or Fe(II) sulfides in deeper regolith. Our results demonstrate the importance of Hg sequestration in deep regolith, below typical soil sampling depths.