Soil‐gas diffusivity in large soil monoliths

Abstract

The variability of gas diffusion in soil is not well known, but is important for assessing greenhouse gas emissions, soil decontamination, oxidation in soil and plant and root respiration. The goal of this study was to assess small-scale variability of the relative soil-gas diffusivity (D s / D o , m soil air m −1 soil ) using large intact soil monoliths and to compare D s / D o calculation methods. Neon (Ne) was maintained constant at the lower boundary of three monoliths of two soils (a sand and an organic soil). Ne concentration was measured at large spatial and temporal frequencies. Calculation methods included the use of average concentration, and average D s / D o per horizon, per section, or for the entire soil profile. Considering all sections of the monoliths, D s / D o varied from 3.5 × 10 −3 to 1.2 × 10 −1 for the A p horizon and from 4.8 × 10 −3 to 8.3 × 10 −1 for the B f horizon in the sand and from 1.0 × 10 −3 to 7.9 × 10 −3 for the O hp horizon and from 2.4 × 10 −4 to 7.7 × 10 −2 for the O f horizon in the organic soil. For the entire soil profile, variations in D s / D o between monoliths reached 125% in the sand and 56% in the organic soil. The D s / D o calculation method influenced the apparent variability (CV) of D s / D o and, to a lesser extent, D s / D o values of the overall soil profile. Differences in D s / D o between monoliths could not be explained solely by the variability of total soil porosity and air-filled porosity. Soil macroporosity (cracks and earthworm burrows) and layering greatly influenced variability of gas movement. Thus, the choice of sampling procedure, calculation method and modelling must be governed by the scale of the processes of interest and soil variability attributes.

Multiscale spatial variability of CO2 emissions and correlations with physico-chemical soil properties

“…(4) Convective flux and gas movement in cracks and burrows should be considered for further development of predicting tools of gas movement toward the atmosphere (Lange et al, 2009). • The sampling design could be improved by taking into account changes in soil series, soil layering, and topography since they affect small and large scale spatial distributions.…”

“…Variability in D s / D o and CO 2-flux also depends on soil cracking and earthworm burrows (macropores). Large pores increase the movement of all gases by at least one order of magnitude Deurer et al, 2009) at small scale (Lange et al, 2009). The soils, although sandy, showed large, deep (10 cm) and frequent cracks during spring time, especially at site 1 (Fig.…”

“…The soil at site 1 was composed of two soil series, the Brébeuf series (Canadian classification: Orthic Humo-ferric Podzol (Soil Classification Working Group, 1998), International classification: Gleyic Podzol (IUSS, 2006) in most of the field and the Pontiac series (Canadian classification: Orthic Dystric Brunisol (Soil Classification Working Group, 1998), International classification: Cambisol Dystric (IUSS, 2006) in the East/ South-East part of the field (Lamontagne and Nolin, 1997;Raymond et al, 1976). At site 1, the first horizon (0.00 to 0.25 m ± 0.05 m) of the Brébeuf series was sandy loam (A p ) with many cracks, earthworm and insect burrows, and fine roots (Lange et al, 2009). The second horizon (0.25 m ± 0.05 m to 0.40 m ± 0.05 m) was a compacted sand (B f ) containing only few roots and a lot of concretions (Fig.…”

A Modified Method for the In Situ Measurement of Soil Gas Diffusivity

“…Normalized, the relative apparent diffusion coefficient (Eq. [2]) becomes independent from the sample gas and only depends on soil properties (Penman, 1940;Lange et al, 2009). Hence, it should be equal for both measured gases.…”

“…To account for the natural spatial variability due to water distribution and preferential flow paths, Allaire et al (2008) recommended the measurement volume to be preferably large. Recently, Lange et al (2009) tried to overcome this problem by measuring gas diffusion in large soil monoliths. The authors assessed the spatial variability of gas diffusion and noted that preferential flow paths like earthworm burrows or plant roots strongly contribute to gas diffusion in soils.…”