X-ray CT-derived soil characteristics explain varying air, water, and solute transport properties across a loamy field

The characterization of soil pore space geometry is important for explaining fluxes of air, water, and solutes through soil and understanding soil hydrogeochemical functions. X‐ray computed tomography (CT) can be applied for this characterization, and in this study CT‐derived parameters were used to explain water, air, and solute transport through soil. Forty‐five soil columns (20 by 20 cm) were collected from an agricultural field in Estrup, Denmark, and subsequently scanned using a medical CT scanner. Nonreactive tracer leaching experiments were performed in the laboratory along with measurements of air permeability (Ka) and saturated hydraulic conductivity (Ksat). The CT number of the matrix (CTmatrix), which represents the moist bulk density of the soil matrix, was obtained from the CT scans as the average CT number of the voxels in the grayscale image excluding macropores and stones. The CTmatrix showed the best relationships with the solute transport characteristics, especially the time by which 5% of the applied mass of tritium was leached, known as the 5% arrival time (t0.05). The CT‐derived macroporosity (pores >1.2 mm) was correlated with Ka and log10(Ksat). The correlation improved when the limiting macroporosity (the minimum macroporosity for every 0.6‐mm layer along the soil column) was used, suggesting that soil layers with the narrowest macropore section restricted the flow through the whole soil column. Water, air, and solute transport were related with the CT‐derived parameters by using a best subsets regression analysis. The regression coefficients improved using CTmatrix, limiting macroporosity, and genus density, while the best model for t0.05 used CTmatrix only. The scanning resolution and the time for soil structure development after mechanical activities could be factors that increased the uncertainty of the relationships. Nevertheless, the results confirmed the potential of X‐ray CT visualization techniques for estimating fluxes through soil at the field scale

Estimating the universal scaling of gas diffusion in coarse-textured soils

Gas diffusion, D, in partially saturated soils, constitutes a critical topic in soil sciences. However, it is a complex process and this limits its characterization and estimation. In this study, we analyzed and parameterized the soil gas diffusion using a combination of percolation theory (PT) and the effective-medium approximation (EMA). Here, we selected 126 coarse-textured soils with measurements including sand, silt, and clay content, bulk density, organic matter, porosity, soil water content measured at different pressure heads and saturation-dependent gas diffusion. First, we adopted the van Genuchten model, fit it to the soil water retention curve (SWRC), optimized its parameters, and determined the water content at the inflection point. Second, the parameters of the universal scaling law from PT and EMA were optimized by directly fitting the model to the saturation-dependent gas diffusion data. Those parameters are (1) the critical air-filled porosity, εc, (2) the crossover air-filled porosity, εx, at which the gas movement behavior changes from the percolation theory domain to the EMA domain; and (3) the average pore coordination number, z. Next, a multiple linear regression analysis (MLRA) was applied to link εc, εx and z to other soil parameters, such as soil textural and/or hydraulic properties. Uncertainties in our results were evaluated using a jack-knife resampling technique, which involved applying the MLRA more than 7000 times. Results revealed that the most accurate estimations were obtained when both soil textural and hydraulic properties were used simultaneously. However, the use of only soil textural parameters presents practical advantages, as it provides excellent estimations for εx and z, although not for εc. The latter is a critical parameter in the application of the PT and EMA to gas diffusion that requires both the soil basic properties and water saturation curve properties to be correctly estimated.This work was partially supported by the Spanish Ministry of Science, Innovation, and Universities [grant numbers RTI2018-099052-BI00 and PID2022-139990NB-I00]

Effects of Soil Compaction and Organic Carbon Content on Preferential Flow in Loamy Field Soils

Abstract: Preferential flow and transport through macropores affect plant water use efficiency and enhance leaching of agrochemicals and the transport of colloids, thereby increasing the risk for contamination of groundwater resources. The effects of soil compaction, expressed in terms of bulk density (BD), and organic carbon (OC) content on preferential flow and transport were investigated using 150 undisturbed soil cores sampled from 15 × 15–m grids on two field sites. Both fields had loamy textures, but one site had significantly higher OC content. Leaching experiments were conducted in each core by applying a constant irrigation rate of 10 mm h−1 with a pulse application of tritium tracer. Five percent tritium mass arrival times and apparent dispersivities were derived from each of the tracer breakthrough curves and correlated with texture, OC content, and BD to assess the spatial distribution of preferential flow and transport across the investigated fields. Soils from both fields showed strong positive correlations between BD and preferential flow. Interestingly, the relationships between BD and tracer transport characteristics were markedly different for the two fields, although the relationship between BD and macroporosity was nearly identical. The difference was likely caused by the higher contents of fines and OC at one of the fields leading to stronger aggregation, smaller matrix permeability, and a more pronounced pipe-like pore system with well-aligned macropores