Abstract
A model that couples the diffusion of O2 to plant roots at the microscale to diffusion of O2 through the soil at the macroscale is derived. The solution is complex, but if the length scales (Zr and Zm; see APPENDIX for list of symbols) are equated a simple analytical expression can be obtained. This model is used to investigate relationship between a critical air-filled porosity (θm) and the other parameters; viz temperature (T), O2 concentration in the bulk soil (C*), O2 concentration at the root surface (Cr), root length density (L), the ratio of root radius (a) to the water film radius (R), microbial respiration (Mo), and length scales (Zm and Zr) related to the depth to which microbial and plant respiration are active in the model using sensitivity analysis. The model shows that θm is not very sensitive to the O2 concentration at the root surface (Cr), or the ratio of root radius (a)/water film radius (R), but is sensitive to all the other parameters in some part of their range. The results indicate that indices used to define soil aeration; O2 diffusion rate (ODR) or O2 flux, O2 concentration, or air-filled porosity, which have been previously used, are related and a single critical value for these is unlikely. If a constant critical value exists for one of these indexes it cannot exist for the other two. It is also shown that it is highly unlikely that a universal critical parameter related to soil aeration exists for any of these parameters. It is concluded that more parameters than ODR, O2 concentration, or air-filled porosity need to be measured if progress in soil aeration research is to be made.
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Claims 6
Claim · model-written summary
The modeled critical air-filled porosity was weakly sensitive to root-surface oxygen concentration and the root-radius to water-film-radius ratio.
Not reportedDirection
Verbatim
t0“Plant Roots”
e0“θm is not very sensitive to the O2 concentration at the root surface (Cr), or the ratio of root radius (a)/water film radius (R)”
Claim · model-written summary
The modeled critical air-filled porosity was sensitive to the remaining analyzed parameters over some of their ranges.
Not reportedDirection
Verbatim
t0“Plant Roots”
e1“but is sensitive to all the other parameters in some part of their range”
Claim · model-written summary
The model indicated that oxygen diffusion rate or flux, oxygen concentration, and air-filled porosity are related indices with no likely shared single critical value.
Not reportedDirection
Verbatim
t0“Plant Roots”
e2“O2 diffusion rate (ODR) or O2 flux, O2 concentration, or air-filled porosity, which have been previously used, are related and a single critical value for these is unlikely”
Claim · model-written summary
If one soil aeration index has a constant critical value, the other two cannot also have one.
Verbatim
t0“Plant Roots”
e3“If a constant critical value exists for one of these indexes it cannot exist for the other two”
Claim · model-written summary
A universal critical soil aeration parameter was judged highly unlikely.
Verbatim
t0“Plant Roots”
e4“it is highly unlikely that a universal critical parameter related to soil aeration exists for any of these parameters”
Claim · model-written summary
The authors recommend measuring more parameters than oxygen diffusion rate, oxygen concentration, or air-filled porosity to advance soil aeration research.
Verbatim
t0“Plant Roots”
e5“more parameters than ODR, O2 concentration, or air-filled porosity need to be measured if progress in soil aeration research is to be made”