A Growth Model of Root Mass and Vertical Distribution, Dependent on Carbon Substrate from Photosynthesis and with Non-limiting Soil Conditions

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Abstract
A mechanistic root growth model is developed, in which it is assumed that water and nutrients are readily available and are non-limiting to the growth process. Growth is controlled by photosynthate supply, which is parameterized in terms of the concentration of carbon substrate; it is assumed that there is an upper limit to the root density – that is, the structural weight of root per unit soil volume. Horizontal homogeneity is assumed throughout. Newly synthesized root structure is placed in a transient pool of mobile root material, from which material is returned to the spatially fixed root structure with first-order kinetics. Root penetration into uncolonized soil horizons is achieved by ascribing a diffusion-like process to the mobile material. Carbon substrate transport occurs by movement down, and proportional to, the concentration gradient within the fixed root. The two pseudo-diffusion coefficients, along with parameters for the maximum rooting density, and the availability and utilization of carbon substrate, determine the time course of root growth and the development of the root profile. The model includes processes such as maintenance and growth respiration which utilize carbon substrate and also the effects of senescence on root structure. Realistic results with exponential-type root distributions with depth are obtained. A range of parameter values is considered, and the response of the model to changing the availability of photosynthate is examined.

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