Yadi Wang, PhD

Weathering profiles are often complex, extending from more highly transformed materials in the near surface to less weathered parent material at depth. It is difficult to resolve from field data the impacts of material properties on the short-term rates of mineral weathering when different depths of the profile are reacted with aggressive meteoric waters. In the present study, we aimed to measure variation in mineral transformation reactions that occurs under controlled laboratory conditions… Show more

Weathering profiles are often complex, extending from more highly transformed materials in the near surface to less weathered parent material at depth. It is difficult to resolve from field data the impacts of material properties on the short-term rates of mineral weathering when different depths of the profile are reacted with aggressive meteoric waters. In the present study, we aimed to measure variation in mineral transformation reactions that occurs under controlled laboratory conditions for samples collected as a function of depth across a deep weathering profile in volcaniclastic parent rock. We conducted a series of batch weathering experiments of extracted core materials from two borings to 35 m across a zero-order catchment in the rhyolitic Jemez River Basin Critical Zone Observatory, NM, USA. Upon reaction with aqueous solutions pre-equilibrated with atmospheric CO2, mineral dissolution was not limited to one phase, but included a combination of reactions. Mineral transformation rates were dependent on the mineral assemblage, texture, and legacy of hydrothermal alteration. Results also indicated an important role of existing and neo-formed colloids in Al, Si, and Fe mobilization and redistribution, especially for materials with evidence of previous hydrothermal alteration. This study highlights the use of experimental weathering of extracted cores to help interpret field-based, hydrochemistry with an approach that may be employed in other geologically complex terrains. Show less

Microbial dynamics drive the biotic machinery of early soil evolution. However, integrated knowledge of microbial community establishment, functional associations, and community assembly processes in incipient soil is lacking. This study presents a novel approach of combining microbial phylogenetic profiling, functional predictions, and community assembly processes to analyze drivers of microbial community establishment in an emerging soil system. Rigorous submeter sampling of a basalt-soil… Show more

Microbial dynamics drive the biotic machinery of early soil evolution. However, integrated knowledge of microbial community establishment, functional associations, and community assembly processes in incipient soil is lacking. This study presents a novel approach of combining microbial phylogenetic profiling, functional predictions, and community assembly processes to analyze drivers of microbial community establishment in an emerging soil system. Rigorous submeter sampling of a basalt-soil lysimeter after 2 years of irrigation revealed that microbial community colonization patterns and associated soil parameters were depth dependent. Phylogenetic analysis of 16S rRNA gene sequences indicated the presence of diverse bacterial and archaeal phyla, with high relative abundance of Actinomyceles on the surface and a consistently high abundance of Proteobacteria (Alpha, Beta, Gamma, and Delta) at all depths. Despite depth-dependent variation in community diversity, predicted functional gene analysis suggested that microbial metabolisms did not differ with depth, thereby suggesting redundancy in functional potential throughout the system. Null modeling revealed that microbial community assembly patterns were predominantly governed by variable selection. The relative influence of variable selection decreased with depth, indicating unique and relatively harsh environmental conditions near the surface and more benign conditions with depth. Additionally, community composition near the center of the domain was influenced by high levels of dispersal, suggesting that spatial processes interact with deterministic selection imposed by the environment. These results suggest that for oligotrophic systems, there are major differences in the length scales of variation between vertical and horizontal dimensions with the vertical dimension dominating variation in physical, chemical, and biological features. Show less

Colloid transport through complex and dynamic hydrologic systems is rarely studied, owing to the difficulty of constraining initial and boundary conditions and quantifying colloid-porous media and colloid-colloid interactions in transient flow systems. Here we present a particle tracer experiment conducted on a sloped lysimeter receiving periodic rainfall events for 10 days. Four unique, DNA-labelled particle tracers were injected both in sequence and in parallel, together with a conservative… Show more

Colloid transport through complex and dynamic hydrologic systems is rarely studied, owing to the difficulty of constraining initial and boundary conditions and quantifying colloid-porous media and colloid-colloid interactions in transient flow systems. Here we present a particle tracer experiment conducted on a sloped lysimeter receiving periodic rainfall events for 10 days. Four unique, DNA-labelled particle tracers were injected both in sequence and in parallel, together with a conservative tracer (deuterium), over the course of the first day and allowed to move through the system. Discharge-particle tracer concentration curves and the spatial distribution of particle tracer mass retained in the soil at the end of the experiment were found to be highly dependent on the timing of the tracer injection and the precipitation input and subsequent dynamic response of the water table. Overall, DNA-labelled particle tracer 4 was similar to deuterium and decreased over time with the exception of a few peaks later in the experiment. The individual particle tracer breakthrough curves suggest a complex system with different fast transport mechanisms and slow retention-release mechanisms, which resulted in particle tracers transferring faster than deuterium in the first 10 h of the experiment but being exceeded by deuterium soon after deuterium started to breakthrough. The experiment not only highlights the interaction of repeated colloidal pollution events in hydrologic systems with different pre-event saturation conditions, but also the benefits of using multiple synchronous or sequential tracer applications to dissect explicit formulations of water flow and colloid transport processes in complex and dynamic hydrological systems. Such explicit process formulations could help improve understanding hydrologically-controlled transport through catchments and the quantitative prediction of these processes with water quality models. Show less

Understanding the process interactions and feedbacks among water, porous geological media, microbes, and vascular plants is crucial for improving predictions of the response of Earth’s critical zone to future climatic conditions. However, the integrated coevolution of landscapes under change is notoriously difficult to investigate. Laboratory studies are limited in spatial and temporal scale, while field studies lack observational density and control. To bridge the gap between controlled… Show more

Understanding the process interactions and feedbacks among water, porous geological media, microbes, and vascular plants is crucial for improving predictions of the response of Earth’s critical zone to future climatic conditions. However, the integrated coevolution of landscapes under change is notoriously difficult to investigate. Laboratory studies are limited in spatial and temporal scale, while field studies lack observational density and control. To bridge the gap between controlled laboratory and uncontrollable field studies, the University of Arizona built a macrocosm experiment of unprecedented scale: the Landscape Evolution Observatory (LEO). LEO comprises three replicated, heavily instrumented, hillslope-scale model landscapes within the environmentally controlled Biosphere 2 facility. The model landscapes were designed to initially be simple and purely abiotic, enabling scientists to observe each step in the landscapes’ evolution as they undergo physical, chemical, and biological changes over many years. This chapter describes the model systems and associated research facilities and illustrates how LEO allows for tracking of multiscale mater and energy fluxes at a level of detail impossible in field experiments. Initial sensor, sampler, and soil coring data are already providing insights into the tight linkages between water low, weathering, and microbial community development. These interacting processes are anticipated to drive the model systems to increasingly complex states and will be impacted by the introduction of vascular plants and changes in climatic regimes over the years to come. By intensively monitoring the evolutionary trajectory, integrating data with mathematical models, and fostering community-wide collaborations, we envision that emergent landscape structures and functions can be linked, and significant progress can be made toward predicting the coupled hydro-biogeochemical and ecological responses to global change. Show less

Distributions of water transit times (TTDs), and related storage-selection (SAS) distributions, are spatially-integrated metrics of hydrological transport within landscapes. Recent works confirm that the form of TTDs and SAS distributions should be considered time-variant—possibly depending, in predictable ways, on the dynamic storage of water within the landscape. We report on a 28-day periodic-steady-state-tracer experiment performed on a model hillslope contained within a 1-m3 sloping… Show more

Distributions of water transit times (TTDs), and related storage-selection (SAS) distributions, are spatially-integrated metrics of hydrological transport within landscapes. Recent works confirm that the form of TTDs and SAS distributions should be considered time-variant—possibly depending, in predictable ways, on the dynamic storage of water within the landscape. We report on a 28-day periodic-steady-state-tracer experiment performed on a model hillslope contained within a 1-m3 sloping lysimeter. Using experimental data, we calibrate physically-based, spatially-distributed flow and transport models, and use the calibrated models to generate time-variable SAS distributions, which are subsequently compared to those directly observed from the actual experiment. The objective is to use the spatially-distributed estimates of storage and flux from the model to characterize how temporal variation in water storage influences temporal variation in flow-path configurations, and resulting SAS distributions. The simulated SAS distributions mimicked well the shape of observed distributions, once the model domain reflected the spatial heterogeneity of the lysimeter soil. The spatially-distributed flux vectors illustrate how the magnitude and directionality of water flux changes as the water-table surface rises and falls, yielding greater contributions of younger water when the water-table surface rises nearer to the soil surface. The illustrated mechanism is compliant with conclusions drawn from other recent studies, and supports the notion of an inverse-storage effect, whereby the probability of younger water exiting the system increases with storage. This mechanism may be prevalent in hillslopes and headwater catchments where discharge dynamics are controlled by vertical fluctuations in the water-table surface of an unconfined aquifer. This article is protected by copyright. All rights reserved. Show less

Studying co-evolution of hydrological and biogeochemical processes in the subsurface of natural landscapes can enhance the understanding of coupled Earth-system processes. Such knowledge is imperative in improving predictions of hydro-biogeochemical cycles, especially under climate change scenarios. We present an experimental method, designed to capture sub-surface heterogeneity of an initially homogeneous soil system. This method is based on destructive sampling of a soil lysimeter designed to… Show more

Studying co-evolution of hydrological and biogeochemical processes in the subsurface of natural landscapes can enhance the understanding of coupled Earth-system processes. Such knowledge is imperative in improving predictions of hydro-biogeochemical cycles, especially under climate change scenarios. We present an experimental method, designed to capture sub-surface heterogeneity of an initially homogeneous soil system. This method is based on destructive sampling of a soil lysimeter designed to simulate a small-scale hillslope. A weighing lysimeter of one cubic meter capacity was divided into sections (voxels) and was excavated layer-by-layer, with sub samples being collected from each voxel. The excavation procedure was aimed at detecting the incipient heterogeneity of the system by focusing on the spatial assessment of hydrological, geochemical, and microbiological properties of the soil. Representative results of a few physicochemical variables tested show the development of heterogeneity. Additional work to test interactions between hydrological, geochemical, and microbiological signatures is planned to interpret the observed patterns. Our study also demonstrates the possibility of carrying out similar excavations in order to observe and quantify different aspects of soil-development under varying environmental conditions and scale. Show less

Transit times through hydrologic systems vary in time, but the nature of that variability is not well understood. Transit times variability was investigated in a 1 m3 sloping lysimeter, representing a simplified model of a hillslope receiving periodic rainfall events for 28 days. Tracer tests were conducted using an experimental protocol that allows time-variable transit time distributions (TTDs) to be calculated from data. Observed TTDs varied with the storage state of the system, and the… Show more

Transit times through hydrologic systems vary in time, but the nature of that variability is not well understood. Transit times variability was investigated in a 1 m3 sloping lysimeter, representing a simplified model of a hillslope receiving periodic rainfall events for 28 days. Tracer tests were conducted using an experimental protocol that allows time-variable transit time distributions (TTDs) to be calculated from data. Observed TTDs varied with the storage state of the system, and the history of inflows and outflows. We propose that the observed time variability of the TTDs can be decomposed into two parts: ‘internal' variability associated with changes in the arrangement of, and partitioning between, flow pathways; and ‘external' variability driven by fluctuations in the flow rate along all flow pathways. These concepts can be defined quantitatively in terms of rank StorAge Selection (rSAS) functions, which is a theory describing lumped transport dynamics. Internal variability is associated with temporal variability in the rSAS function, while external is not. The rSAS function variability was characterized by an ‘inverse storage effect', whereby younger water is released in greater proportion under wetter conditions than drier. We hypothesize that this effect is caused by the rapid mobilization of water in the unsaturated zone by the rising water table. Common approximations used to model transport dynamics that neglect internal variability were unable to reproduce the observed breakthrough curves accurately. This suggests that internal variability can play an important role in hydrologic transport dynamics, with implications for field data interpretation and modeling. This article is protected by copyright. All rights reserved. Show less

The contribution of neo-formed secondary colloids to discharge water can significantly interfere the hydrogeochemical dynamic and responses and cause poor understanding of catchment behavior and landscape characteristics. This study investigated the impact of colloidal fraction (0.45 µm filtrates), separated from the truly dissolved fraction (0.025 µm filtrates) in discharge water, to the concentration-discharge (C-Q) relation and the hysteresis as well as to the geochemical thermodynamic… Show more

The contribution of neo-formed secondary colloids to discharge water can significantly interfere the hydrogeochemical dynamic and responses and cause poor understanding of catchment behavior and landscape characteristics. This study investigated the impact of colloidal fraction (0.45 µm filtrates), separated from the truly dissolved fraction (0.025 µm filtrates) in discharge water, to the concentration-discharge (C-Q) relation and the hysteresis as well as to the geochemical thermodynamic modeling of mineral stability with respect to secondary phase precipitation under a hot dry-wet transition and a cold humid seasons on three identical large-scale crushed basalt filled hillslopes during an incipient process. Colloid mobilization largely exhibited during dry-wet transition period for all analyzed elements (Si, Al, Mg, Ca, P, Fe and Mn) but only pronounced for transition metals (Fe and Mn) during the humid season. Comparing 0.45 µm, C-Q relation in 0.025 µm filtrates for Mg remained chemostatics trend while Si, Fe and Mn appeared chemodynamic behavior and Al, Ca and P exhibited a dilution effect . Si, P and Fe showed different source distributions between two filtrates in discharge water as opposite hysteresis rotations was provided. While Al, Mg, Ca and Mn do not have clear change in hysteresis rotation, the magnitude and direction of raising and falling limb flushing indices for each element response differently between filtrates in both seasons. Under the dry-wet transition period, saturation indices of allophane-like poorly crystalline (except proto-imogolite) was overestimated to supersaturation condition in 0.45 µm filtrates but solution appeared to be in equilibrium with respect to minerals in 0.025 µm filtrates. More stable forms of kaolinite and Mg-montmorillonite were partially in equilibrium in 0.025 µm filtrates indicated the occurrence of Ostwald ripening even in early-stage of basalt weathering. Show less