Ecohydrological Recovery Theme:

The primary forests of the granitic-based Southern Piedmont were supported by deep, well- to moderately-well drained soils often with substantial O horizons and deep A and E horizons with high infiltration rates, hydraulic conductivities, and rapid drainage.  Macropore networks connected aboveground atmospheres with the lowest depths of the CZ even through clayey B horizons in which the pore network depended on deep rooting and re-rooting, the activity of soil macro-invertebrates, and aggregation facilitated by organo-clay complexes. Historic agricultural erosion transformed Piedmont CZs not only by eroding and removing O, A, E, and even some B horizons but by destroying macropore networks, leaving behind contemporary secondary forests with diminished net primary productivities supported by thin surface soils and clayey B and B/C horizons in which macroporosity is only slowly reforming.  Gully systems that incised deep channels the B and C horizons continue to convey large fractions of annual runoff and suspended sediments, reducing recharge to the deep CZ environments and groundwaters.

Biogeochemical Decoupling Theme:

Despite acidic, highly weathered soils, the primary forests of the Southern Piedmont had relatively high net primary productivities (NPP) due to warm and humid climates, deep rooting, and tight biogeochemical recirculation of nutrients.  The subsurface CZ was weathered deeply especially by high concentrations of biogenic CO2 that diffused downward through macropore networks.  The pore networks allowed other gases such as CH4, 222Rn, and O2 and solutes to exchange across great depths.  However, with land degradation disrupting the pore networks, regenereating secondary forest roots mainly in clayey B and C horizons only slowly re-aggregated and re-developed macropore space.  The overall biogeochemical impact of historic agriculture has tended to decouple, even to isolate surficial layers of the belowground CZ from those at depth.

Erosion-Induced Carbon Dynamics Theme:

Soil organic matter is destabilized and greatly diminished by accelerated decomposition when CZs are cutover and soils plowed.  However, not all agricultural losses of soil organic matter from a degraded soil derive from oxidation and return to the atmosphere as CO2.  Lateral fluxes of eroded soil organic matter can be deposited down slope or downstream and subsequently buried in alluvial fans and terraces along streams and rivers, i.e., in low lying, moist environments with potentially low decomposition rates (Billings et al., 2010, van Oost, 2007).  Whether a significant fraction of eroded soil organic matter is preserved in these depositional sites has significant consequences for estimates of global carbon cycling (Harden et al., 1999; Billings et al., 2010; van Oost et al., 2007).  Furthermore, the rate at which eroded upland sites are re-accumulating contemporary soil organic C (SOC) may be accelerated relative to that pre-disturbance, since erosion has exposed iron- and aluminum-oxide rich B horizons that may have considerable C stabilization potential.

Human-CZ Interactions Theme:

Across much of the Southern Piedmont, historic and on-going changes in the region’s CZs interact with human livelihoods, adaptations, and governance.  Human uses and alteration of the natively productive, yet fragile, CZ transformed not only the CZ itself, but humanity and its on-going relations with the land.  This proposal’s Human-CZ Interactions theme has two fronts.  The first is motivated by the fact that environmental history—a study of humanity’s long-term, varied impact on CZ evolution—is at the heart of this Calhoun CZO proposal.  Legacies of historical changes in the Piedmont’s CZ continue to interact with human management, regional ecosystem services, and governance of these environments. We suggest that the history of the Piedmont is ripe for reinterpretation, specifically from a perspective that brings together environmental history and CZ evolution.  The second front will work to bring CZO science into local land-use policy and decision-making. Traditional methods of sharing scientific information are inadequate for communicating with public-policy and decision-makers (Lemos et al., 2012).  We will build on methods being developed at the Coweeta LTER by Co-PI Nelson and colleagues, to develop frameworks that link the sciences and the public (Heynen et al. submitted).

Dynamic Persistence of Alternative States Theme:

Despite >50 years of reforestation, contemporary CZs are impeded in their recovery of hydrologic and biogeochemical process because of positive feedback effects of altered states.  As recovery of ecosystem is hampered by due to exposure of clayey soil horizons, reduced infiltration rates, and the persistence of deeply incised gullies and stream channels.  This implies that these post-agricultural landscapes may be recovering on time scales much longer than a secondary forest can regrow.  The altered state (the eroded degraded landscape) may persist through the stabilizing effects of such feedbacks.  Should revegetation of the degraded land mask these feedbacks, evapotranspiration may have increased, but the highly altered catchments may persist with collapsed macropore networks that are only slowly recovering and upper CZ components effectively decoupled from those at depth.