Soil characteristics play a key role in modeling nutrient competition in plant communities

Soil and nutrient properties, via their influence on nutrient diffusion rates in the soil, may play a key role in determining the outcome of plant competition for nutrients. We used two models to explore the potential contributions of nutrient uptake kinetics, root density, soil properties, and nutrient type to interspecific plant competition for soil nutrients. The first model uses well-known nutrient diffusion and absorption relationships to generate soil nutrient concentration maps and nutrient uptake at the scale of individual roots (PARIS-M). A second model (PARIS-E) was developed based on a fit of the Hill equation to the output of PARIS-M. The PARIS-E model provides an accurate and simple means of determining the relative contributions of sink strength (root surface area X uptake kinetics) vs. space occupation (number of roots per unit area) to competition at equilibrium as modeled by PARIS-M. An analysis based on these two models suggests the following: (1) Diffusive supply (soil nutrient buffer capacity X effective diffusion coefficient) determines the relative importance of space occupation vs. sink strength for nutrient competition. (2) At the low range of reported values of diffusive supply, competition depends on space occupation and, therefore, the species with the most roots per unit area is the most competitive. Sink strength gains in importance as diffusive supply increases and dominates competitive interactions at the high end of the range of reported diffusive supplies. (3) The relative importance of space occupation vs. sink strength depends primarily on soil water content and soil texture, because diffusive supply is sensitive to these factors. Diffusive supply is relatively insensitive to nutrient type. This analysis suggests that nutrient competition models should include the effects of soil properties as a determinant of the relative contributions of sink strength vs. space occupation.