Grazed pastures are recognized as a dominant source of nitrous oxide (N2O), a highly potent greenhouse gas. Studies have examined soil physical controls on N2O emissions, including soil moisture status. Limited attempts to link N2O emissions with soil‐diffusivity (Dp/Do), using repacked soil cores, have shown peak N2O emissions to align with a relatively narrow window of Dp/Do, despite a relatively wide range in water‐filled pore space (WFPS), across a range of soil bulk densities. Such detailed studies have not been performed with intact soil cores. We investigated the effects of soil‐water characteristic (SWC) and Dp/Do on N2O emissions from intact soil samples, retrieved at three depths (0–5, 5–10, 10–15 cm) from three perennial pasture sites that received a KNO3 solution (1800 mg, N mL−1). We observed distinct fingerprints of SWC and Dp/Do, which showed clear effects of soil structure on diffusion‐controlled gas emissions. Depth‐wise variation in soil moisture diminished as the soil was subjected to higher matric potential (> ∼ ‐100 kPa). Variation in Dp/Do, was more pronounced in the dry soil (> ∼ ‐1000 kPa), being largely constrained by soil moisture in wet soil (∼ ‐100 kPa) with little depth‐wise variation. Measured N2O fluxes peaked within narrow ranges of WFPS and Dp/Do, 0.90–0.95 and 0.005–0.01, respectively. The value of Dp/Do can be determined using parametric models and presents a pasture management (e.g., irrigation, soil physical disturbance such as pasture renovation and animal treading)) tool to minimize N2O emissions: soil Dp/Do should be maintained above a range of 0.005–0.01 to minimize N2O emissions. Core Ideas Peak N2O fluxes from intact soil cores occurred when diffusivity ranged from 0.005–0.01 This peak N2O flux diffusivity range was 0.005–0.01 regardless of soil or depth (0–15 cm) The diffusivity range for peak N2O flux equalled that observed in repacked soil cores Diffusivity values are readily determined and add to the suite of soil management tools

Grazed pastures and cultivated fields are significant sources of greenhouse gas (GHG) emissions, in particular N 2 O emissions derived from fertilizer deposition and animal excreta. Net surface emissions rely on subsurface gas transfer controlled mainly by diffusion, expressed as the soil-gas diffusivity (D p /D o). The value of D p /D o is a function of soil air-filled porosity (ε) and gaseous phase tortuosity (Ʈ), both of which vary with soil physical properties including soil texture and structure. Agricultural soils are often structurally aggregated and characterized by two distinct regions (inter-and intra-aggregated pores), however, such soils are subjected to frequent compaction and tillage resulting in alteration to structural arrangement. In this study, a comparative analysis between the Currie (1960) and Taylor (1949) methods was performed to provide a computational insight into selecting an appropriate method for calculating D p /D o in agricultural soils. Currie's (1960) method was chosen for further analysis of the soils in this study. Results show that the D p /D o in aggregated soil cannot be expressed using a simple linear, power law or combined linear and power law functions due to the presence of two-region characteristics. A new "Two-Region model" was developed to parameterize the D p /D o of aggregated soils, and tested against repacked samples from two Sri Lankan agricultural soils. This Two-Region model clearly distinguished tortuosity effects on gas movement with respect to density and textural variations within and between aggregates and outperformed previous models. The fitting parameters (α 1 , α 2 , β 1 and β 2) varied correspondingly with soil density, and the weighting factor (w) clearly distinguished the boundary between the two regions (inter-and intra-aggregates) of structured soils. The model developed will be of interest to those seeking to model the diffusion of GHG emissions and gas exchange between the atmosphere and soils. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Grazed pasture constitutes a major source of agriculturally derived nitrous oxide (N2O), which is a potent greenhouse gas. Soil texture and structure, soil moisture, and soil‐gas diffusivity are considered to be major soil physical drivers controlling pastoral N2O emissions. Research gaps exist regarding their combined roles on N2O emissions dynamics. This study used 2‐mm‐sieved and repacked soil samples, retrieved at three depths (0, 10, and 15 cm) from three grazed pasture sites in New Zealand, to investigate the combined effects of soil‐water characteristic (SWC) and soil‐gas diffusivity on N2O emissions. The existing and modified parametric functions were used to numerically characterize and parameterize measured particle size distribution (PSD), SWC, and soil‐gas diffusivity. We observed distinct PSDs within the three soils with little variation across depths. Distinct fingerprints were observed for SWC and gas diffusivity in the three pasture soils, suggesting clear effects of soil type on diffusion‐controlled gas emissions. The soil moisture retention above approximately −10 kPa decreased with increasing depth and showed clear soil type effects. Soil‐gas diffusivity, on the other hand, showed pronounced depth‐wise variation below approximately −1000 kPa. Pore tortuosity was found to be nonlinearly correlated to air‐filled pore space as well as the PSD. The measured N2O fluxes peaked around a diffusivity window of 0.005 to 0.01 for all soil types, and the corresponding water‐filled pore space ranged from 0.80 to 0.95. The results provide guidance for managing pasture soils to reduce high N2O fluxes through reductions in compaction and excess irrigation such that the critical diffusivity window, where peak N2O emissions occur, is avoided.

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Characterization of differently textured porous materials, as well as different volumetric porous media mixtures, in relation to mass and heat transport is vital for many engineering and research applications. Functional relations describing physical (e.g., grain-size distribution, total porosity), thermal, and gas diffusion properties of porous media and mixtures are necessary to optimize the design of porous systems that involve heat and gas transport processes. However, only a limited number of studies provide characterization of soil physical, thermal, and gas diffusion properties and the functional relationships of these properties under varying soil water contents, especially for soil mixtures, complicating optimization efforts. To better understand how mixing controls the physical, thermal, and gas diffusion properties of porous media, a set of laboratory experiments was performed using five volumetric mixtures of coarse-and fine-grained sand particles. For each mixture, the grain-size distribution (GSD), thermal conductivity, and gas diffusivity were obtained and parameterized using existing and suggested parametric models. Results show that the extended, two-region Rosin-Rammler particle-size distribution model proposed in this study could successfully describe the bimodal behavior of the GSD of binary mixtures. Further, the modified Côté and Konrad thermal conductivity model adequately described the thermal conductivity-water saturation relations observed in different mixtures. The proposed simple soil-gas diffusivity descriptive model parameterized the upper limit, average, and lower limit behavior in gas diffusivity-air content relations in apparently texture-invariant gas diffusivity data. Results further show a close analogy between gas diffusivity and thermal conductivity and their variation with saturation across different binary mixtures. Overall, the results of the study provide useful numerical insight into the physical, thermal, and gas transport characteristics of binary mixtures, with wide implications for future engineering and research applications that involve multicomponent porous systems. . 2018. Characterization of grainsize distribution, thermal conductivity, and gas diffusivity in variably saturated binary sand mixtures. Vadose Zone J. 17:180026.