The critical zone (CZ) extends from the bottom of the weathered bedrock to the top of the tree canopies. CZ processes and fluxes, such as weathering, nutrient and carbon cycling are critical for the provision of critical ecosystem services that support life on the planet. CZ function is regulated by its internal structure, as inferred for example from concentration-discharge (C-Q) relationships. On the other hand, CZ fluxes are slowly and constantly shaping its features, as seen for example in rates of mineral weathering across climatic gradients or the coevolution of soils, vegetation and topography across an elevation gradient. Understanding the relationships between CZ function and structure becomes fundamental for enhancing our capacity to predict and mitigate impacts from short term environmental disturbances to medium and long-term alterations imposed by climate change.
This PhD dissertation explores the interplay between structural development and hydrologic behavior and response of the CZ. My objective was to provide empirical understanding of the following question: How water fluxes and states modulate and are subject to changes in CZ structure. In order to pursue this goal, I’ve taken advantage of the experimental set-up hosted at Landscape Evolution Observatory (LEO) - Biosphere 2, which allows for a high degree of observability and control. The system under study consists of a 1 m3 sloping lysimeter filled with fresh un-weathered crushed basalt. Due to its slope, the lysimeter can be taken as a model of a hillslope, a landscape unit in which most biogeochemical interactions occur.
In the first chapter of this dissertation, I address the role of hydrology in shaping the incipient heterogeneity. Throughout 2 years, the lysimeter experienced sequences of hydrologic inputs (irrigation) and drying periods under bare soil conditions, leading to detectable biogeochemical patterns. Here, my main question was how observed geochemical weathering states and microbial diversity and abundance are influenced by hydrologic behavior within two years of imposed wet-dry cycles. For this purpose, an intensive sampling procedure was undertaken for the assessment of the incipient heterogeneity within the system, followed by modelling of the hydrologic history summarized in terms of moisture states, cumulative fluxes and average residence times of water. I’ve found significant imprints of hydrologic behavior on both biological and geochemical patterns of heterogeneity, which suggests a common framework to assess how heterogeneity develops in incipient systems. More specifically, the amount of time spent by water within the subsurface appeared, or the residence time (RT) appeared to be the main control on observed geochemical states and spatial distribution of different fila of microorganisms.
The second and third chapter combined represent an effort to experimentally observe residence times of water within the subsurface. Chapter two presents a method to directly observe the transport of solute within the model system using Electrical Resistivity Tomography (ERT). For that, I have equipped the lysimeter with electrodes to be used with an ERT acquisition system, allowing me to obtain high frequency temporal images of soil resistivity along 5 cross-sections of the lysimeter. The tracking of solute movement through ERT was based on the estimation of the spatial distribution of fluid electrical conductivity (EC) within the lysimeter. It is important to note that the estimation of the spatial distribution of EC has traditionally been a challenge in ERT studies at field and laboratory scale, especially under transient conditions. The presented method has therefore the potential to be applied at different settings and find uses beyond the scope of the estimation of RT.
The third chapter of this dissertation deals with the extension of an existing theory on the estimation of transit time distributions (TTD) within the context of controlled experimentation. Transit times of water (TT) are of great importance in hydrologic sciences, since they are related to the very basic question of the fate of water once it reaches the landscape. The theory of TTD represents a lumped-systems approach towards the understanding of TT at natural landscapes and has a long history of application in both natural as well as artificial systems. However, due to its lumped nature of system representation, TTD theory does not explicitly address the internal variability of flow pathways within the subsurface and therefore benefit from understanding of the mechanistic basis that they represent. The study presented in chapter 3 introduces a method for the estimation of the varying ages of water within the lysimeter under a prescribed hydrologic forcing allowing for an approach towards the time-variability of TT
