Land-use legacy dynamics in decades- and centuries-old soils

Abstract

This dissertation asks how anthropogenic disturbances are subsequently modified by pedogenic processes over century and decadal-time scales in two soil systems that have, at best, a modicum of previous study. The first system is a bottomland floodplain with legacy sediment in a low-gradient Piedmont watershed that has experienced a century of pedogenesis following accelerated sedimentation. The second is the urban soil of a mid-sized city in the Piedmont, where the elapse of half a century has transformed soil lead concentrations from gasoline and lead-based paint. This dissertation broadens the focus of land-use legacies in the Southern Piedmont and its results bear upon decisions in floodplain restoration and soil Pb exposure.The dynamics of bottomland floodplains laden with legacy sediments are interrogated over two chapters. Chapter 2 constrains the timing of legacy sediment deposition with radioactive isotopes and demonstrates that legacy sediment soils limit accumulation of mineral-associated soil carbon, suggesting changes to the floodplain regime since sediment deposition. Chapter 3 asks whether legacy sediments drive loss of floodplain function by resolving belowground dynamics of soil moisture and redox regimes over 18 months of field-level measurements. These findings shed light on the mechanisms by which widespread legacy sediment deposition has transformed Piedmont floodplains to be drier and more aerobic. Urban soil Pb legacies are examined in Chapter 3, which conducts the first city-wide sampling of soil Pb in North Carolina. This work reconciles the immobility of soil Pb, reflected in resolvable patterns still present today, with the mobility and decreasing concentrations of soil Pb enabled by human activities and soil redistribution processes. Overall, these findings show how soil processes transform environmental legacies from human-driven disturbances over time and contribute to pedology’s pursuit of centuries- and decades-scale soil change.