Browsing by Subject "Soil sciences"

This study seeks to investigate the dynamics of dead plant carbon over fifty years of old-field forest development at the Calhoun Long Term Soil-Ecosystem Experiment (LTSE) in South Carolina, USA. Emphasis is on the transition phase of the forest, which is less well studied than the establishment and early thinning phase or the steady state phase. At the Calhoun LTSE, the biogeochemical and ecosystem changes associated with old field forest development have been documented through repeated tree measurements and deep soil sampling, and archiving of those soils, which now allow us to examine changes that have occurred over the course of forest development to date.

In this dissertation, I first quantify the accumulation of woody detritus on the surface of the soil as well as in the soil profile over fifty years, and estimate the mean residence times of that detrital carbon storage. Knowing that large accumulations of C-rich organic matter have piled onto the soil surface, the latter chapters of my dissertation investigate how that forest-derived organic carbon has been incorporated into mineral soils. I do this first by examining concentrations of dissolved organic carbon and other constituents in soil solutions throughout the ecosystem profile and then by quantifying changes in solid state soil carbon quantity and quality, both in bulk soils and in soil fractions that are thought to have different C sources, stabilities, and residence times. To conclude this dissertation, I present the 50-year C budget of the Calhoun LTSE, including live and dead plant carbon pools, to quantify the increasing importance of detrital C to the ecosystem over time.

This exceptional long term soil ecosystem study shows that 50 years of pine forest development on a former cotton field have not increased mineral soil carbon storage. Tree biomass accumulated rapidly from the time seedlings were planted through the establishment phase, followed by accumulations of leaf litter and woody detritus. Large quantities of dissolved organic carbon leached from the O-horizons into mineral soils. The response of mineral soil C stocks to this flood of C inputs varied by depth. The most surficial soil (0-7.5cm), saw a large, but lagged, increase in soil organic carbon (SOC) concentration over time, an accumulation almost entirely due to an increase of light fraction, particulate organic matter. Yet in the deepest soils sampled, soil carbon content declined over time, and in fact the loss of SOC in deep soils was sufficient to negate all of the C gains in shallower soils. This deep soil organic matter was apparently lost from a poorly understood, exchangeable pool of SOM. This loss of deep SOC, and lack of change in total SOC, flies in the face of the general understanding of field to forest conversions resulting in net increases in soil carbon. These long term observations provide evidence that the loss of soil carbon was due to priming of SOM decomposition by enhanced transpiration, C inputs, and N demand by the growing trees. These results suggest that large accumulations of carbon aboveground do not guarantee similar changes below.

Rivers interact with their valleys from headwaters to mouth, but nowhere as dynamically as in their floodplains. Rivers deliver water, sediments, and solutes onto the floodplain land surface, and the land in turn supplies solutes, leaves, and woody debris to the channel. These reciprocal exchanges maintain both aquatic and terrestrial biodiversity and productivity. In this dissertation I examine river-floodplain exchanges on the well-studied Nyack Floodplain, a dynamic, gravel-bedded floodplain along the Middle Fork Flathead River in the mountains of northwest Montana. I quantify exchanges at multiple timescales, from moments to centuries, to better understand how connectivity between aquatic and terrestrial habitats shapes their ecology.

I first address connectivity in the context of a long-standing question in ecosystem ecology: What determines the rate of ecosystem development during primary succession? Rivers have an immediate effect on floodplains when scouring floods remove vegetation and nutrients such as nitrogen (N) and leave only barren soils, but they might also affect the ensuing primary succession through the gradual delivery of N and other materials to floodplain soils. I quantify N inputs to successional floodplain forest soils of the Nyack Floodplain and find that sediment deposition by river flood water is the dominant source of N to soils, with lesser contributions from dissolved N in the river, biological N fixation, and atmospheric deposition. I also synthesize published rates of soil N accumulation in floodplain and non-floodplain primary-successional systems around the world, and I find that western floodplains often accumulate soil N faster than non-floodplain primary successional systems. My results collectively point to the importance of riverine N inputs in accelerating ecosystem development during floodplain primary succession.

I next investigate the role of river-floodplain exchanges in shaping the spatial distribution of a suite of soil properties. Even after flood waters have receded, dissolved N, carbon (C), and moisture could be delivered from the river to floodplain soils via belowground water flow. Alternatively, C inputs and N withdrawals by floodplain vegetation might be a dominant influence on soil properties. To test these hypotheses, I excavated and sampled soil pits from the soil surface to the water table (50-270 cm) under forests, meadows, and gravel bars of the Nyack Floodplain. Near-surface soils had C and N pools and N flux rates that varied predictably with vegetation cover, but soil properties below ~50 cm reflected influence by neither vegetation cover nor aquifer delivery. Instead, soil properties at these depths appear to relate to soil texture, which in turn is structured by the river's erosional and depositional activities. This finding suggests the revised hypothesis that soil properties in gravel-bedded alluvial floodplains may depend more on the decadal-scale geomorphic influences of floods than on short-term vertical interactions with floodplain vegetation or aquifer water.

Lastly, I explore the potential sources of organic C to the diverse and active community of aquatic organisms in the floodplain aquifer, where the lack of light prohibits in-situ organic C production by photosynthesis. I quantify floodplain carbon pools and the fluxes of organic carbon connecting the aquifer, river, and overlying forest. Spring flood waters infiltrating the soil are responsible for the largest dissolved carbon flux into the aquifer, while very large floods are essential for the other major C input, the burial of woody carbon in the aquifer. These findings emphasize the importance of a dynamic river hydrograph - in particular, annual floods and extreme annual floods - in delivering organic C to the aquifer community.

Overall, this dissertation draws our attention not just to the current exchanges of C, N, water, and sediment but to the episodic nature of those exchanges. To fully understand floodplain ecosystems, we have to consider not just present-day interactions but also the legacies of past floods and their roles in delivering solutes, eroding forests, depositing sediments, and physically shaping the floodplain environment.

Laboratory tests on microscale are reported in which amorphous silica grains were compressed in a liquid environment, namely in solutions with different silica ion concentrations for up to four weeks. Such an arrangement represents an idealized representation of two sand grains. The grain surfaces and asperities were examined in Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) for fractures, silica polymer growth, and polymer strength. Single chains of silica polymers are found to have a failure pulling force of 330 - 450 nN.

A chain of observations are reported for the first time, using Pneumatic Grain Indenter and Grain Indenter-Puller apparatuses, confirming a long-existing hypothesis that a stressed contact with microcracks generates dissolved silica in the contact (asperity) vicinity, which eventually polymerizes, forming a structure between the grains on a timescale in the order of weeks. Such structure exhibits intergranular tensile force of 1 - 1.5 mN when aged in solutions containing silica ion concentrations of 200- to 500 ppm. Stress appears to accelerate the generation of silica polymers around stressed contact regions, so does mica-silica contacts. The magnitude of intergranular tensile force is 2 to 3 times greater than that of water capillary effect between grains.

Laboratory tests on microscale are reported in which two amorphous silica cubes were compressed in a liquid environment, namely in solutions with different silica ion concentrations for up to four weeks. Such an arrangement represents an idealized representation of two sand grains. The grain surfaces and asperities were examined in Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) for fractures, silica gel growth, and polymer strength. In 500ppm solution, silica gel structures a few hundred microns long appeared between stressed silica cubes. In 200ppm solution, silica deposits were found around damaged grain surfaces, while at 90ppm (below silica solubility in neutral pH), fibers a few microns in length were found growing in cube cracks. AFM pulling tests found polymers with strength in the order of 100nN and length between 50 and 100 nm. After aging, size of silica gel is in the order of 10-100 µm with intergranular strength in the order of 0.01-1 mN. We concluded that prolonged compression produced damage in grains, raising local Si ion concentration, and accelerating precipitation, polymerization and gelation of silica on grain surfaces enhancing soil strength at the microscale, hence most likely contributing to the aging phenomenon observed at the macroscale. Mica surfaces near stressed silica contacts were also found to enhance silica gel growth.

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.

This aim of this dissertation is to investigate "pedogenesis" (soil formation and change over multi-millennial timescales with minimal human impact) and "anthropedogenesis" (centurial and decadal soil formation and change through the Holocene with increased human influence) in the highly weathered, upland soils of the Southern Piedmont physiographic region in the southeastern United States. I start by combining an analysis of the cosmogenic nuclide meteoric beryllium-10 (10Be) with a mass balance analysis of pedogenic 9Be loss to estimate how long the Southern Piedmont Ultisol have been residing at Earth's surface. This coupled analysis indicates that pedogenesis has been operating in these highly weathered Ultisols for much, if not all, of the Quaternary; considerably longer than previously thought. Next, I utilize traditional soil analyses alongside iron stable isotope measurements to investigate how one century of reforestation after agricultural land abandonment impacts the coupled carbon -iron cycle in these ancient subsoils. This project suggests that widespread patterns of anthropogenic land use change in the Southern Piedmont have caused significant subsoil changes that impact carbon storage and the distribution of iron deep below ground. Finally, I analyze over 50 years or repeated soil and forest ecosystem observations from the Calhoun Experimental Forest to investigate the relationship between soil macronutrient contents aboveground forest ecosystem development in the region. These long term observations suggest that decadal patterns of secondary forest growth and decline fundamentally alters the role that soils plays in individual ecosystem nutrient cycles and that the potential for ecosystem nutrient loss is highly nutrient dependent, despite well-established ecological theory.