Modeling Nutrient Uptake as a Component of Loblolly Pine Response to Environmental Stress

Abstract

well as accurate, nutrient-uptake models must be combined with detailed whole plant carbon allocation models so that interactions between the two models can occur and the results can describe verifiable changes in key parameters. Model DescriptionsWe employed two nutrient-uptake models, the Barber-Cushman model (Barber, 1984) as modified for thc personal computer by Oates and Barber (1987) and a steady-state model (Yanai, 1994). The two models are similar in approach and share many assumptions with their predecessors (Nye and Spiers, 1964; Nye and Marriot, 1969; Claassen alld Barber, 1976;Cushman, 1979;Barber and Cushman, 1981 ). They simulatc uptake by the average absorbing root in the average soil;there is no consideration of the geometry of the root system or of differences in root properties with age or morphology. The root is esscntially a uniform, linear sink and the amount of soil surrounding the root is defined by the average distance to the next root. In both models, nutrients move toward thc root by both mass now (Ihc movemcnt of solution to the root to support the transpiration stream) and by diffusion along the concentration gradient created by active uptake at the root surface. Uptake at the root surface is described by Michaelis-Menten kinetics.Neither model considers mycorrhizal association, except as it affects the values of parameters in the model, nor root modification of the rhizosphere, except for nutrient depletion.Both modeling approaches have been extensively tested in a number of applications through comparison of model predictions to observed responses. Although.dus is not a fail-safe process, experience has shown that opportunities for error propagation through these modeling approaches lie more with the development of the data used to provide initial values than with the concepts or codes within the models. Consequently, a portion of the discussion in this chpater is devoted to an analysis of how bcst to dcvelop key initial values.The major difference bctween the modcls is that the Barber-Cushman modcl simulates uptake ofa growing root system over a period of time, but without timevarying input. The rate of root growth is one of the values input to the model; that is. it must be spccificd in advance. Similarly.tllere are no inputs of nutrients to tllc soil ~ystem during tIle durution of a model run except as defined by the ability of thc solid phase (C,,) to maintain solution-phase concentration (CJ through buffering. We applied this model with considerable success in a simulation of loblolly pine seedlings for one growing season (Kelly et al., 1992). However, the BarberCushman model as presently configured could not be effectively linked to a plant simulator because it does not accept time-varying input to root growth and soil conditions. We also wanted to allow such feedback as the effect of nutrient uptake on root growth and the effcct of litter quality on nutrient supply. Therefore, in alklition to using the Barr~r-Cu~hman model, we d~veloped a new model to allow a more dynamic ...