Roots Partially in Contact with Soil: Analytical Solutions and Approximation in Models of Nutrient and Water Uptake

Single-root models consider the soil in a cylindrical perspective with roots at the center and assume local homogeneity of the soil matrix. Empirical diffusion coefficients correct for the changes in path length when soils dry out and less pore space is effectively involved (Nye and Tinker, 1977). A complementary view starts at a random position in 3-D belowground space and assumes that transport of water or nutrients will be directed toward the root that generates the strongest concentration gradients, most likely the nearest active root. Common procedures for delineating these in two-dimensional cross-sections of soil are based on a nearest-neighbor Dirichlet tessellation (de Willigen and van Noordwijk, 1987b) and implicitly assume full root–soil contact and roots growing perpendicular to the plane of observation. Root distribution and root–soil contact in the field, however, depend on the way roots interact with soil aggregates and pores (Altemüller and Haag, 1983; van Noordwijk et al., 1993a; Kooistra and van Noordwijk, 1996). Contact between roots and soil is one of the critical issues for progress in soil science as identified by Bouma (2010). While models aimed at quantifying soil–plant relations at the field scale will necessarily have to make simplifying assumptions at the single-root level, the level of bias introduced into standard single-root models by oversimplifying the geometry of the root–soil interface remains a concern. How serious the bias is may depend primarily on the level of compensation that can exist within a root system where individual roots face a range of circumstances. This study dealt with more realistic, complex root–soil geometries and derived ways in which the simple cylindrical model can still be used with appropriate adjustments.

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DOI:

https://doi.org/10.2136/vzj2017.03.0060

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