Maria Ines Dragila

Evaporation from Fractures Exposed at the Land Surface: Impact of Gas‐Phase Convection on Salt Accumulation

A mechanism is investigated by which surface-exposed fractures could be a source of aquifer salinization in low-permeability fractured formations under arid conditions. It is hypothesized that evaporation of pore water within surface-exposed fractures is enhanced by convective air circulation within those fractures that vents moisture to the atmosphere. This evaporation also simultaneously enhances lateral movement of pore water from the adjacent matrix towards the fracture surface, permitting dissolved solutes to precipitate on the surface and form a crust. The salt crust can then dissolve during infiltration events and be flushed downward to the aquifer. Theoretical analysis shows that convective venting is expected during cool nights when atmospheric air is denser than the fracture air. Laboratory experiments support the hypothesis of rapid salt-crust formation in the presence of convectively moving air across a fracture face. A numerical model is developed and used to quantify the buildup of salt on a fracture face.

Effects of gray-tailed vole activity on soil properties.

Abstract Voles are well-known crop pests, especially when peak populations are present, but their role in soil fertility and impacts on agricultural sustainability are not well understood. Five months after the abrupt disappearance of a peak in a gray-tailed vole (Microtus canicaudus) population, we examined burrow structure, determined concentrations of trace elements, carbon and nitrogen in the soil immediately surrounding vole burrows, and compared soil chemical properties to a depth of 90 cm between areas with prior vole activity and areas of no activity. Vole tunneling activity was confined to the top 10 cm of the soil profile and was coincident with the majority of root biomass. Soil NH4+, NO3-, extractable organic carbon, and soil organic matter were greater below vole tunnels than above; however, due to small sample sizes, differences were not significant. There were no differences in trace elements with respect to position around vole tunnels. Vole activity was associated with increased soil nitrate concentrations and decreased soil pH to a depth of 90 cm, indicating that nitrification might be enhanced by vole activity, and that this effect continues after vole populations crash. Greater inorganic nitrogen could have long-term effects on ecosystem productivity. The effects voles have on soil processes that influence carbon and nutrient cycle requires further investigation.

Micro-CT Analysis to Explore Salt Precipitation Impact on Porous Media Permeability

The concern for water scarcity as a future reality for a growing proportion of the planet is driving the need to understand in greater detail the role of water in ecosystem sustainability and resiliency. Pertinent to this conversation is the role that salt precipitation and salinization has on soil resources. This paper addresses this task by presenting results of a detailed study on the reduction in gas permeability of a porous media by subflorescent salt precipitation. Using high-resolution CT scanning and detailed Lattice Boltzman Modeling, the three dimensional distribution of salt was mapped and its effect on permeability was calculated. It was found that salt precipitate mass increases towards the soil surface and that the effect of subsurface precipitation on gas permeability was significant, potentially impacting gas exchange processes between atmosphere and vadose zone. In addition, results from the LBM were compared to estimates using the Koseny-Carman equation. A good correlation between these two methods indicates that the Koseny-Carman equation may be sufficient to calculate changes in permeability, providing a more accessible tool relative to the more complicated LBM approach. This study has made it clear that knowledge of the spatial distribution of salt precipitation is essential for any estimation of gas permeability changes due to subflorescence.

Characterization of Hydrophobic Soils: A Novel Approach Using Mid-Infrared Photoacoustic Spectroscopy

Previous research has produced conclusive evidence to show that soil hydrophobicity is affected by soil organic matter (SOM) and soil water content (WC). Soil hydrophobicity that responds to WC is a unique form of hydrophobicity called reversible hydrophobicity. The mechanistic processes by which water and SOM interact continue to be a subject of investigation. This article presents a novel application of photoacoustic Fourier transform infrared spectroscopy (FT-IR) for the investigation of reversibly hydrophobic soils. Photoacoustic FT-IR data show that, in response to hydration, surface molecules on wetted soil particles interacted differently with mid-infrared radiation than surface molecules present on air-dried soil. This can be interpreted as an effect of the reorienting of amphiphilic molecules in response to hydrationdriven entropic processes. These results suggest that the photoacoustic FT-IR method can be used to elucidate how SOM and water interact at the molecular scale to drive soil hydrophobicity.

The Importance of Advective Fluxes to Gas Transport Across the Earth-Atmosphere Interface: The Role of Thermal Convection

Understanding of gas exchange between the Earth’s upper crust and the atmosphere is vital, as it affects many important processes which concern the water cycle, agricultural activities, greenhouse gas emissions, and more. From a hydrological aspect, water vapor transport is the most important process related to Earth-atmosphere gas exchange, since it affects aboveland water vapor concentration; soil water content; and soil salinity. These three important hydrological parameters respectively affect the global water cycle (Hillel, 1998); water management and agricultural practices; and the formation of salt crusts at and near land surface – which can lead to soil salinization (Weisbrod et al., 2000; Nachshon et al., 2011), an important process from an agricultural point of view. In addition to soil salinization, with respect to agriculture, gas transport in the upper soil profile, i.e., the root zone, is important for soil aeration or movement of oxygen within the soil. Soil aeration is critical for plant root growth, as plants generally cannot get enough oxygen from their leaves (Lambers et al., 2008). Oxygen is not always readily available in the soil pores, since respiration of plants and other organisms and microbial degradation of organic compounds in the ground emit high volumes of CO2 into soil pores, while consuming O2 (Brady, 1999). The exchange rate of air between soils and the atmosphere is crucial to maintain the needed soil aeration and oxygen concentration for plant growth. Since most underground biological activity takes place in the upper parts of the soil profile, the majority of CO2 is formed from the ground surface down to shallow depths of a few meters (Amundson, 2005). As soil temperature and water content increase, the CO2 production increases (Fang & Moncrieff, 1999; Rastogi et al., 2002; Buyanovsky et al., 1986). The increase of organic matter availability will also lead to an increase in CO2 production (Amundson, 2005). For example: Buyanovsky et al. (1986) calculated CO2 production in soil surface cultivated with wheat. Values varied from 4 to 8 g/m d in spring, but in winter as soil temperature dropped below 5°C, CO2 production was reduced to less than 1 g/m d. As for organic matter availability; soil CO2 concentrations at 1 m depth in Tundra, temperate grassland and tropical rain forest are 1000, 7000 and 20,000 ppm, respectively (Amundson, 2005), corresponding to the richness of these soils in organic matter. CO2 concentration in the pores of unsaturated soils, in the range of 3000 ppm is very common for agricultural and grasslands areas (Brady,