Oxygen transport in soil and the vertical distribution of roots

<p>An analytical solution for steady-state oxygen transport in soils including 2 sink terms, <i>viz</i> roots and microbes with the corresponding vertical distribution scaling lengths forming a ratio <i>p</i>, showed <i>p</i> governed the critical air-filled porosity, θ<i>_c</i>, needed by most plants. For low temperature and <i>p</i>, θ<i>_c</i> was <0.1 but at higher temperatures and <i>p</i> = 1, θ<i>_c</i> was >0.15 m³/m³. When root length density at the surface was 10⁴ m/m³ and <i>p</i> > 3, θ<i>_c</i> was 0.25 m³/m³, more than half the pore space. Few combinations of soil and climate regularly meet this condition. However, for sandy soils and seasonally warm, arid regions, the theory is consistent with observation, in that plants may have some deep roots. Critical θ<i>_c</i> values are used to formulate theoretical solutions in a forward mode, so different levels of oxygen uptake by roots may be compared to microbial activity. The proportion of respiration by plant roots increases rapidly with <i>p</i> up to <i>p</i> ≈2.</p>