In summary, soil O2 in a poorly drained pasture decreased with hydrological events such as flooding,
or chemical hydrolysis events following urine deposition. These decreases in soil O2 coincide with
rapid increases in N2O fluxes. It was found that hydrological variables such as WFPS work well to
explain N2O emissions so long as soil properties do not vary. Relative soil gas diffusivity best explains
N2O fluxes regardless of the treatment because it can compensate for how soil properties and soil
moisture interact to influence soil O2 diffusion. These results demonstrate for the first time an O2
diffusion threshold for elevated N2O fluxes in the field, occurring at a value of Dp/Do ≈ 0.006. Further
studies should examine the consistency of this threshold under varying microbial substrate and soil pH conditions.
Acknowledgements
This research was undertaken with CRI Core Funding to Landcare Research from New Zealand's Ministry of Business, Innovation and Employment. We are grateful for the help in the field provided by Neil Smith, and to the Centre for Environment and Soil Research at Lincoln University for the loaned equipment.
Supplementary material
The supplementary data includes a map of the experimental plot (Supplemental Fig. S1), a graphical representation of the spatial distribution of matric potential within the gas collars after surface flooding (Supplemental Fig. S2), a reference to the equations used during this study (Supplemental Table S1), and a reference to the transformations for statistics (Supplemental Table S2).
103
5.7
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110 Supplemental Fig. S2 The distribution of the soil matric potential (Ѱ), which is represented by the numbers in the box, presented from day of the experiment (DOE) 19 to 28. The Ѱ was measured daily from within the gas chambers from the shallow (≈20 mm depth) and deep (≈70 mm depth) measurements.
day of the experiment
19 20 21 22 23 24 25 26 27 28 shallow 0 0 -4 0 -2 0 -1 0 0 0 0 -0 -2 0 -2 0 -2 -1 -3 0 -2 -1 -2 -2 -2 -2 -2 -2 0 0 0 0 -7 -7 -7 -7 -8 -8 -7 -7 -1 0 0 0 -1 0 0 0 0 0 0 0 -6 -2 -2 0 -2 0 0 -4 -6 -4 -2 -2 -2 -1 0 -2 -4 -4 -6 -6 -6 -6 -6 -6 -7 -7 -7 -7 19 20 21 22 23 24 25 26 27 28 deep 0 0 -1 0 -1 0 -2 0 0 0 0 -1 -1 0 0 0 0 -2 -2 0 0 0 -2 0 0 -2 0 -1 -1 -1 0 0 -4 -4 -6 -6 -7 -7 -5 -5 0 0 0 0 -2 -0 0 0 0 0 0 0 0 -1 0 0 -2 0 0 -6 -4 -2 -1 -1 -2 0 -2 -2 -4 -4 -4 -4 -4 -4 -2 -2 -3 -3 -6 -6
111 Supplemental Table S1 The equations used for soil bulk density, total porosity, gravimetric soil moisture, volumetric water content, water-filled pore space, air-filled pore space, and relative soil gas diffusivity
Variable (symbol, units Equation Notes Soil bulk density (ρb, Mg m‐3) 𝜌
𝑏= 𝑀𝑠/𝑉𝑠 Ms is the mass of dry soil (Mg), and Vs is the volume of soil core (m3).
Total porosity (φ, %)
𝛷 = (1 − (𝜌𝑏
𝜌𝑑)) ∗ 100
ρd is the particle density, which is assumed to be 2.65 Mg m-3.
Gravimetric soil moisture (θg) (Blakemore et al. 1987)
𝜃𝑔 = 𝑀𝑆𝑊/𝑀𝑆 MSW is the mass of water
and soil (Mg) Volumetric water content (θv,
m3 m-3) 𝜃𝑣= 𝜃𝑔∗ (
𝜌𝑏
𝜌𝑑) Water-filled pore space
(WFPS, m3 m-3) (Linn and
Doran 1984)
𝑊𝐹𝑃𝑆 =𝜃𝑣
𝛷 Air-filled pore space (ε, m3 m-
3)
ε = 𝛷 − 𝜃𝑣 Relative soil gas diffusivity
(DP/DO, unit less) was
modelled using the Structure- dependent Water-induced Linear Reduction (SWLR) model (Moldrup et al. 2013)
𝐷𝑃 𝐷𝑂= ɛ [1+𝐶𝑚𝛷](ɛ 𝛷) The Cm is a media complexity factor, assumed to be related to soil particle density and therefore bulk density. The Cm for soils with plants is 2.1, which was used during this study Capillary rise equation for
determining the size of soil pores which were water-filled at minimum measured soil matric potential (Ѱ)
𝑟 =0.15 ℎ
Where r is radius of pore (cm), h is the matric potential (cm H2O) (Scott,
112 Supplemental Table S2 Data transformation details
Variable (units) Transform
N2O fluxes Daily N2O emissions (mg-N g-1 m-2) Ln10 (N2O)
Cumulative N2O emissions (mg-N g-1) N/A
Soil chemical data Nitrate (µg g-1 dry soil-1) ʎ = 0.5
Ammonium (µg g-1 dry soil-1) Ln
Hot water carbon (µg g-1 dry soil-1) N/A
Cold water carbon (µg g-1 dry soil-1) ʎ = 0
soil pH (µg g-1 dry soil-1) ʎ = 1
Conductivity (%) Ln
Environmental data Soil temperature 50 mm (°C) ʎ = 1 Soil oxygen with diffusive head 10 mm (°C) ʎ = 5 Soil oxygen with diffusive head 50 mm (°C) ʎ = 5 Soil oxygen with diffusive head 100 mm (°C) ʎ = 5 Soil oxygen without diffusive head 10 mm (°C) ʎ = 5 Soil oxygen without diffusive head 50 mm (°C) ʎ = 5 Soil oxygen without diffusive head 100 mm (°C) ʎ = 5 Volumetric water content 50 mm ʎ = -4 Soil physical data Water filled pore space (m3 m-3) N/A
Soil diffusivity (DP/DO) ʎ = 0.32
Ln, natural log Ln10, log base 10
ʎ, indicates a box cox transformation was used, where the number presented is the optimal lambda
113