Soil Gas-Phase Diffusion
Dr. John Norman
The Precision Agricultural Landscape
Modeling System (PALMS)
Monday, 1:00 – 2:00 pm., 3140 Agronomy
Diffusion
Definition
Mathematical Model: Fick’s Laws
Major Influences: temp., molecular weight,
media it diffuses through
Why Important and examples:
• Rate dependent reactions
•
Gas Diffusion in Soil
Will the diffusion flux differ in soil than bulk air?
YES!...diffusivities will be different
•Fractional volume for transport
•Pores
•Size and distribution
•Air-filled porosity and tortuosity
•Biological
•Plant roots, earthworms
Effective Diffusivity
Fractional volume for transport
Effective Diffusivity in Soil
Pore Shape
Effective Diffusivity in Soil
Pore-Size Distribution
Effective Diffusivity in Soil
Air-filled porosity
Effective Diffusivity in Soil
Tortuosity
Effective Diffusivity in Soil
Tortuosity
Predicting Effective Diffusivity
Air-filled porosity and tortuosity
•Buckingham:
D(ε)/Da = к ε 2
•Water-induced linear reduction
(WLR) Model
D(ε)/Da = ε 1.5 (ε / ϕ)
•Marshall:
D(ε)/Da = ε 1.5
•Penman:
D(ε)/Da = 0.66 ε
Predicting Effective Diffusivity
Air-filled porosity and tortuosity
Moldrup:
D(ε)/Da = k*(ε2.5)/ϕ
But…….
tortuosity is hard to determine
Luo et al. SSSJ. 2008. No-till silt loam soil in Pennsylvania under corn,
soybean, and alfalfa rotation
Predicting Effective Diffusivity
• Estimate
• Assumptions in equations
• Quick and Cheep
Measuring Effective Diffusivity
• Actual values
• May need many measurements to represent site
• Timely, Laborious, and Expensive
Measuring Soil as Diffusivity
Laboratory Analysis
Measuring Soil as Diffusivity
Laboratory Analysis
Ln [(C – Cs)/(C0 – Cs)]
• Soil core of known dimensions
• Measure O2 vs. time
• Calculate apparent gas diffusivity
Dp = apparent diffusivity
t = time
(-Dp α2)/ε
ε = air-filled porosity
C = gas chamber O2 conc.
Cs = ambient O2 conc.
C0 = initial gas chamber O2 conc.
αl = 0.895(hl)^(0.4599)
l = core length
time
h=ε/a
a = gas chamber volume per cross-sectional area
Rolston, D.E. and P. Moldrup. 2002. Gas Diffusivity. Methods of Soil
Analysis. SSSA. No. 5 Part 4. p. 1113 – 1139.
Soil-gas exchange
Which Dominates?
Advection
Diffusion
Measuring
Atmospheric Gas Exchange
In-situ Analysis
Measuring
Atmospheric Gas Exchange
HEADS UP!!!!
When and Where?
Dynamic system
•Diurnal temperature cycle (soil breathing)
•Atmospheric pressure changes and wind
pumping
•Soil water content and spatial variability of Ф
•Biological activity (Source/Sink)
•Roots (type, growth stage, distribution, etc.)
•Microorganisms (type, food source, etc.)
Class Over