Paleosols are formed when the topsoil gets buried by the lateral distribution of soil and can store large quantities of soil organic matter (SOM) that may persist over millennial timescales due to its detachment from the disturbances at the surface. We studied buried SOM dynamics in the Brady paleosol, a deep loess (aeolian) deposit in Nebraska, USA, where climate has historically driven varying rates of loess deposition during the late Pleistocene and Holocene, burying soils up to 50m below the surface. Soils were sampled along the depositional and erosional transects at depths from 0.2 to 5.5m to understand the variability in the physical and chemical composition of the soils. We used elemental and isotopic compositions of C, N, 13C, and 15N, along with radiocarbon, base cation concentrations, and Fourier Transformed Infrared Spectroscopy (FTIR) to determine the distribution, stabilization, and composition of SOM and organic carbon in the soil profiles. Our results show a general decreasing trend of 13C and 15N values with depth, suggesting root input to soil carbon pools and the presence of less decomposed SOM in the deeply buried soil layers. Radiocarbon analysis of bulk soil indicated a loss of ancient carbon and incorporation of new photosynthate carbon in the eroding transect. Our stabilization study indicated, modern and buried Brady soil both are flocculated in physical structure and Brady soils has more monovalent cations compared to the modern soils. To determine the vulnerability of the SOM to the addition of moisture, we added water to soil from the different transects and depths at 60% pore space capacity in two different experimental setups – repeated wet and dry cycles and continuously wet. We found that repeated wetting and drying led to higher CO2 efflux for buried Brady soils from erosional transects compared to the modern soils collected from the depositional transects. Our findings indicate that change in soil moisture status can play a critical role in destabilizing previously protected ancient carbon. Finally, our study highlights the need for furthering our understanding of how a predicted increase in precipitation quantity and intensity coupled with accelerated erosion can release large quantities of greenhouse gases by mineralization of previously protected old carbon stocks.

Soil aggregate degradation by medium and high intensity fires is often attributed to loss of soil organic matter, whereas low intensity fires are often considered benign to soil aggregate degradation because organic and inorganic binding agents are relatively stable at these low temperature burns. Because of this, there are limited studies specifically focusing on the effect of low intensity fires on soil aggregation. However, recent long-term studies have reviewed that aggregate breakdown can occur long periods of time after low intensity burns. The aggregate breakdown could not be explained by loss of soil binding agents. Previous studies have indirectly tested the hypothesis that stress exerted by rapid vaporization of soil pore water during low intensity burns could lead to aggregate breakdown in the long term. I further explore this mechanism of soil aggregate degradation in a forest sandy loam and shrubland loam soil. In this work, I (1) provide direct proof of the hypothesis by measuring the pressure inside individual moist soil aggregates burned at 175°C and (2) show that soil aggregate degradation by this mechanism can expose physically protected soil organic carbon to decomposition. The pore pressure increase in both the forest and the shrubland aggregates increased with increasing soil water content. Furthermore, tensile strength of both types of soil aggregates decreased with increase in soil water content, suggesting that moist soil aggregates may be more susceptible to breakdown by this mechanism. Additionally, I provide a model predicting that strain on aggregates from rapidly vaporized soil water is maximized in initially wetter soil aggregates. The degradation of soil aggregates by rapidly vaporized soil pore water was observed in forest soils that experienced increase in rate of decomposition of soil organic following low intensity fires. Increase in decomposition of soil organic carbon in shrubland soil aggregates were observed for all burn treatments, and was linked to increase in dissolved organic carbon likely due to loss of cytoplasmic organic compounds from lysis of soil microbes. The results provided in this thesis suggest that low intensity burns can negatively affect soil aggregation in ways that were not studied before since low intensity burns are often considered benign to soil aggregation, and that more research should focus on low intensity fires’ effects on soil aggregates as the number of these burns have been increasing over the last few decades.

Emulsions appear in many subsurface applications including bioremediation, surfactant-enhanced remediation, and enhanced oil-recovery. Modeling emulsion transport in porous media is particularly challenging because the rheological and physical properties of emulsions are different from averages of the components. Current modeling approaches are based on filtration theories, which are not suited to adequately address the pore-scale permeability fluctuations and reduction of absolute permeability that are often encountered during emulsion transport. In this communication, we introduce a continuous time random walk based alternative approach that captures these unique features of emulsion transport. Calculations based on the proposed approach resulted in excellent match with experimental observations of emulsion breakthrough from the literature. Specifically, the new approach explains the slow late-time tailing behavior that could not be fitted using the standard approach. The theory presented in this paper also provides an important stepping stone toward a generalized self-consistent modeling of multiphase flow.

Soil structure directly determines important soil physical properties including porosity, hydraulic conductivity, water retention, and mechanical strength. It also indirectly influences almost all biological and chemical processes that occur in soil. Conversely, the development, stability, and dynamics of soil structure are dictated by the very physical, chemical, and biological processes that occur within the structured soil. There is ample empirical evidence showing the effectiveness of wetting and drying in the presence of organic matter in soil aggregation and stabilization. However, the mechanisms that bond the particles together under this process need more investigation. The goal of this dissertation was to understand and develop quantitative description of the role of wetting and drying cycles in presence of exudates in the formation and stabilization of soil aggregates within the rhizosphere.

In this dissertation, I (a) developed a new, easy and rapid method to measure the carbohydrate and total carbon concentrations using UV spectrophotometry, (b) examined whether the association between plant root and bacteria exudates with neutral sand particles occurred and defined the mechanism of this association, (c) developed a conceptual/mathematical model describe the soil aggregation mechanism in presence of exudates under multiple wetting and drying cycles, (d) examined the mechanisms that affect rhizosphere water dynamics and whether these dynamics are a result of the osmotic potential induced by root exudates or the soil structure modification that occurred because of these exudates, and (e) developed a mathematical model to quantitatively describe the experimental results of the effect of water potential induced by root exudates on water evaporation rate.

This dissertation presented a framework for in-depth understanding on how wetting and drying cycles in the presence of exudates promote soil aggregation and stabilization within the rhizosphere. It also advanced our understanding of the benefits of presence of root exudates in the rhizosphere on water retention and evaporation rate and provided the right-scale physics for high resolution computational modeling of water dynamics around the plant roots and root water uptake.

The transferability of excess wasted organic materials, such as agricultural and forestry residues into materials like biochar and compost, to be used as a nutrient-rich organic soil amendment for food production has been viewed as an ecological approach to enhance soil ecosystem services. In this dissertation a variety of research techniques and experiments were used to investigate the effects of different locally produced biochar and also biochar dairy manure co-composts on soil hydrological properties, GHG emissions, nitrogen leaching, and crop productivity. The dissertation is divided into three main research chapters. In the first chapter, we generated moisture retention curves for seven biochar derived from wasted orchard materials e.g. almond shell, walnut shell, and almond pruning, using a mobile pyrolizer unit. More specifically, we used the water sorption film approach to determine specific surface area for the different biochar and incorporated the data into a model to assess its influence on soil moisture content. Additionally, three of the biochar (one from each feedstock) were also selected and used in two 109 days incubation studies to investigate biochar influence on soil respiration under different moisture levels. In the second chapter, numerical simulation was performed (5-years impact) on the three selected biochar to assess its impacts on soil hydrological properties and plant uptake dynamic for common cropping systems with dissimilar irrigation practices. Lastly, in the third chapter the three selected biochar were co-composted with fresh dairy manure for 45 days then used as soil amendment in a 133 day outdoor soil-tomato column study. Leaf chlorophyll content, canopy coverage, and GHG measurements were taken throughout the season as proxy for crop productivity and soil emissions as influenced by soil treatments. Results from chapter one show that walnut shell biochar has the greatest surface area while almond shell derived biochar has the most positive effect on moisture retention and soil respiration. Next, our 5-years numerical simulation shows that application of biochar at 5% enhanced water conservation by reducing seasonal soil evaporation loss and allowing for more root water uptake. However this positive effect varied between cropping systems and is substantially greater in the rainfed compared to irrigated system. In the last chapter, results from our soil-tomato study show greater positive effect on soil ecosystem services e.g. nitrogen retention and crop productivity, from biochar-dairy manure co-compost soil treatments compared to the control. However crop yield was constrained by external factors such as plant water stress.