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1. Evaluation of Dual Domain Mass Transfer in Porous Media at the Pore Scale

   https://doi.org/10.1111/gwat.13328  

   Dorchester, Leland; Day‐Lewis, Frederick D.; Singha, Kamini (March 2024, Groundwater)

   Dual-porosity models are often used to describe solute transport in heterogeneous media, but the parameters within these models (e.g., immobile porosity and mobile/immobile exchange rate coefficients) are difficult to identify experimentally or relate to measurable quantities. Here, we performed synthetic, pore-scale millifluidics simulations that coupled fluid flow, solute transport, and electrical resistivity (ER). A conductive-tracer test and the associated geoelectrical signatures were simulated for four flow rates in two distinct pore-scale model scenarios: one with intergranular porosity, and a second with an intragranular porosity also defined. With these models, we explore how the effective characteristic-length scale estimated from a best-fit dual domain mass transfer (DDMT) model compares to geometric aspects of the flow field. In both model scenarios we find that: (1) mobile domains and immobile domains develop even in a system that is explicitly defined with one domain; (2) the ratio of immobile to mobile porosity is larger at faster flow rates as is the mass-transfer rate; and (3) a comparison of length scales associated with the mass-transfer rate (Lα) and those associated with calculation of the Peclet number (LPe) show LPe is commonly larger than Lα. These results suggest that estimated immobile porosities from a DDMT model are not only a function of physically mobile or immobile pore space, but also are a function of the average linear pore-water velocity and physical obstructions to flow, which can drive the development of immobile porosity even in single-porosity domains. 

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2. Iron isotope systematics of shale-derived soils as potentially influenced by small mineral particle loss

   Bern, Carleton; Yesavage, Tiffany; Pribil, Michael (September 2017, Proceedings of the 12th International Symposium on Applied Isotope Geochemistry (AIG-12))

   Loss of small mineral particles from soil has been suggested as a process that can produce net isotopic fractionation in the remaining soil. We extracted water dispersible colloids (WDCs) from bulk soil collected at the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) and measured their Fe isotopic composition for comparison to published data from the site. The goal was to explain soil δ56Fe values that become lighter as Fe is lost from soil. The range of δ56Fe values for WDCs was 0.22 to 0.59 ‰, barely intersecting the value of ~0.8 ± 0.3‰ predicted by mass balance for particulate Fe loss by a previous study. The WDCs extracted likely represent a mixture of unfractionated Fe inherited from shale minerals and secondary Fe fractionated by weathering zone processes. Thus, although the WDC compositions do not confirm small mineral particle losses as causing overall Fe isotope fractionation in SSHO soils, they are compatible with that interpretation. 

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3. Nanofibrous mat-enabled soil water absorption for continuous soil nitrate sensing

   https://doi.org/10.1016/j.compag.2025.110319  

   Haridass, Vignesh Kumar; Castellano, Michael J; Dong, Liang (July 2025, Computers and Electronics in Agriculture)

   Continuous monitoring of soil nitrate levels is essential for effective soil nutrient management. However, limited soil pore water at low soil water content levels hinders miniaturized soil sensor surfaces from efficiently interacting with nutrient ions. To address this, we introduce a nanofibrous mat designed to enhance nitrate detection by increasing connectivity between miniature sensors and the soil solution. Composed of polysulfone, polymethylmethacrylate, and polyvinyl alcohol, this mat is fabricated using electrospinning and electrospray methods to balance water absorbency, mechanical durability, and ease of manufacturing. When wrapped around an ion-selective electrode-based nitrate sensor, the mat improves access to soil pore water, acts as a filter, prevents direct sensor-soil particle contact, and reduces the impact of soil particle surface charges on sensor measurements. Continuous nitrate monitoring with both mat-wrapped and bare sensors was conducted in controlled and field environments. Linear regression analysis indicates that the mat-wrapped sensor has a stronger correlation with conventional salt extract methods for measuring soil nitrate levels. T-tests confirm statistically significant differences between sensor measurements and the salt extraction method. Additionally, Bland-Altman analysis reveals that mat-wrapping reduces mean bias and narrows the limits of agreement, demonstrating improved agreement with the conventional method. Notably, the mat-wrapped sensor performs consistently across varying soil moisture conditions. These findings suggest that the water-absorbing mat improves the ability of the sensor to monitor nitrate continuously by accommodating varying soil moisture levels over time, making the mat-wrapped soil nitrate sensor a viable improvement for in-field measurements of soil solution chemistry. 

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4. Effects of aridity and vegetation on plant-wax δD in modern lake sediments

   https://doi.org/10.1016/j.gca.2010.06.018  

   Polissar, Pratigya J; Freeman, Katherine H (October 2010, Geochimica et Cosmochimica Acta)

   We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (εa) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of 2H and 18O associated with evaporation of lake waters. The most negative εa values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum εa values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0–2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ18O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau. 

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5. Changes in soil pore structure generated by the root systems of maize, sorghum and switchgrass affect in situ N2O emissions and bacterial denitrification

   https://doi.org/10.1007/s00374-023-01761-1  

   Lucas, Maik; Gil, J.; Robertson, G. P.; Ostrom, N. E.; Kravchenko, A. (August 2023, Biology and Fertility of Soils)

   Abstract Due to the heterogeneous nature of soil pore structure, processes such as nitrification and denitrification can occur simultaneously at microscopic levels, making prediction of small-scale nitrous oxide (N 2 O) emissions in the field notoriously difficult. We assessed N 2 O+N 2 emissions from soils under maize ( Zea mays L .) , switchgrass ( Panicum virgatum L.), and energy sorghum ( Sorghum bicolor L.), three potential bioenergy crops in order to identify the importance of different N 2 O sources to microsite production, and relate N 2 O source differences to crop-associated differences in pore structure formation. The combination of isotopic surveys of N 2 O in the field during one growing season and X-ray computed tomography (CT) enabled us to link results from isotopic mappings to soil structural properties. Further, our methodology allowed us to evaluate the potential for in situ N 2 O suppression by biological nitrification inhibition (BNI) in energy sorghum. Our results demonstrated that the fraction of N 2 O originating from bacterial denitrification and reduction of N 2 O to N 2 is largely determined by the volume of particulate organic matter occluded within the soil matrix and the anaerobic soil volume. Bacterial denitrification was greater in switchgrass than in the annual crops, related to changes in pore structure caused by the coarse root system. This led to high N-loses through N 2 emissions in the switchgrass system throughout the season a novel finding given the lack of data in the literature for total denitrification. Isotopic mapping indicated no differences in N 2 O-fluxes or their source processes between maize and energy sorghum that could be associated with the release of BNI by the investigated sorghum variety. The results of this research show how differences in soil pore structures among cropping systems can determine both N 2 O production via denitrification and total denitrification N losses in situ. 

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