An analytical solution for steady-state oxygen transport in soils including 2 sink terms, viz roots and microbes with the corresponding vertical distribution scaling lengths forming a ratio p, showed p governed the critical air-filled porosity, θc, needed by most plants. For low temperature and p, θc was <0.1 but at higher temperatures and p = 1, θc was >0.15 m3/m3. When root length density at the surface was 104 m/m3 and p > 3, θc 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 θc 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 p up to p ≈2.
Synthesis of vertical root biomass density, L [= L0 exp(–z/Zr), z is the depth positive down (m)] (m/m3), data using an exponential function to represent the distribution suggested that, on average, 70 ± 10% of fine roots in 10 terrestrial biomes were located in the upper 0.1 m of soil. Integrated over the root-zone, LT is given by the product of the function’s 2 parameters, the surface value of L, L0 (m/m3), and length scale, Zr (m). As postulated, negative correlations were obtained between L0 and Zr. For a maize (Zea mays L.) crop, significantly different distributions were measured during relatively dry and wet seasons and predicted by our model. For woody and herbaceous plants, Zr (the value determines the rate of decrease in L with depth) averaged 0.3 and 0.2 m, respectively, while the corresponding averages for Rm0 [= L0.ρr, ρr is root density (kg/m)] were 2.7 and 1.1 kg/m3.
Analyses of the Vertical Distribution of Roots in Wheat, Soybean and Rice in Tilled and Non-tilled Multipurpose Paddy Fields.