Soil‐Gas Diffusivity and Soil‐Moisture effects on N₂O Emissions from Intact Pasture Soils

Abstract

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

Denitrification in soil as a function of oxygen availability at the microscale

“…Diffusivity is mainly controlled by soil bulk density and water saturation (Balaine et al, 2013;Klefoth et al, 2014). These empirical models predict diffusivity based on empirical relationships with total porosity ( ) and air-filled porosity (ε) (Millington and Quirk, 1961;Moldrup et al, 2000;Resurreccion et al, 2010;Deepagoda et al, 2011Deepagoda et al, , 2019. As expected the discrepancy between calculated D emp and simulated D sim was highest at water saturation > 75 % WFPS, where discontinuity due to packing procedure took full effect as described earlier (Figs.…”

“…The dependence of denitrification on diffusion constraints was demonstrated by several models that were developed to predict the formation of anoxic centres within soil aggregates (Greenwood, 1961;Arah and Smith, 1989;Arah and Vinten, 1995;Kremen et al, 2005). The distance threshold for anoxic conditions to emerge was set on an ad hoc basis at 5 mm from connected air at the end of incubation but is likely to vary with O 2 demand by local microbial activity (CO 2 release represented by the green fringe area, item 2) during the incubation (Kremen et al, 2005;Rabot et al, 2015;Ebrahimi and Or, 2018;Keiluweit et al, 2018;Kravchenko et al, 2018;Schlüter et al, 2019). Because we could only conduct X-ray CT scans at the end of incubation, redistribution of water during the incubation time cannot be ruled out.…”

Irrigation Scheduling with Soil Gas Diffusivity as a Decision Tool to Mitigate N2O Emissions from a Urine-Affected Pasture

“…Balaine et al [20] showed that relative gas diffusivity (D p /D o ; where D p is the soil gas diffusion coefficient (m 3 soil air m −1 soil s −1 ) and D o is the gas diffusion coefficient in free air (m 2 air s −1 )) was better than WFPS for identifying the threshold of N 2 O production when comparing soils across a range of bulk densities and soil moistures. This was further confirmed using repacked soil cores, intact soil cores and in situ [21][22][23]. A D p /D o value ≤ 0.02 indicates the onset of anaerobic soil conditions [24], while a value of 0.006 has been shown to result in peak N 2 O emissions [20].…”

Gas‐Diffusivity based characterization of aggregated agricultural soils

“…Balaine et al, 2013;Owens et al, 2017), thus recognizing the potential of D p /D o as a key predictor for soil conditions that are conducive for N 2 O fluxes. Recent studies have also indicated the presence of a critical diffusivity window (D p /D o ∼ 0.005-0.01), in both intact and repacked pasture soils, which yields a peak in N 2 O emissions regardless of the soil texture, structure, and soil moisture status (Chamindu Deepagoda et al, 2019). Calculation of D p /D o essentially requires solving the classical Fick's laws of diffusion under specified boundary conditions.…”