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DIVISION S-1—SOIL PHYSICS

Oxygen Transport to Plant Roots

Modeling for Physical Understanding of Soil Aeration

^a CSIRO, Land and Water, 120 Meiers Road, Indooroopilly QLD 4068, Australia and CRC for Sustainable Sugar Production
^b CSIRO, Land and Water, P.O. Box 1666, Canberra ACT 2601, Australia

* Corresponding author (freeman.cook{at}csiro.au)

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 (Z_r and Z_m; 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 (_m) and the other parameters; viz temperature (T), O₂ concentration in the bulk soil (C*), O₂ concentration at the root surface (C_r), root length density (L), the ratio of root radius (a) to the water film radius (R), microbial respiration (M_o), and length scales (Z_m and Z_r) related to the depth to which microbial and plant respiration are active in the model using sensitivity analysis. The model shows that _m is not very sensitive to the O₂ concentration at the root surface (C_r), 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.

Abbreviations: NLWR, nonlimiting water range • ODR, O₂ diffusion rate

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