Modeling resource interactions under multiple edaphic stresses

The majority of plants in natural and agricultural ecosystems experience multiple nutrient stresses concurrently. Empirical investigation of plant responses to multiple stresses is laborious, especially when the adaptive value of numerous plant traits such as root architectural traits are considered. Functional-structural plant modeling permits the investigation of these complex relationships in silico, allowing many scenarios and hypotheses to be evaluated. We implemented procedures to estimate the effect of multiple plant stresses in SimRoot, a dynamic root growth model which accounts for mass balances in the entire plant. The model was run for the three primary plant nutrients nitrogen (N), phosphorus (P), and potassium (K) and the combinations of P-K and P-N. In about 10% of the simulations of P-K limitation we found significant interactions between P and K uptake consistent with the predictions of the “multiple limitation hypothesis” (MLH) in which plants balance resources so that not one single resource is limiting growth. In the majority of cases of P-K limitation one nutrient dominated growth responses, in agreement with the Liebig-Sprengel “law of the minimum” (LM). In 34% of the simulations of P-N limitation we found evidence for dual resource limitation, but these were not strictly consistent with MLH in that the P and N stresses manifested at different times during plant development. Our results are consistent with empirical studies showing that neither of the dominant paradigms for plant response to multiple resource constraints, MLH or LM, are adequate. New models must be developed that take into account biological factors determining the nature of such responses, including temporal dynamics of contrasting resources and root plasticity responses which may favor one resource at the expense of others.