The dependence of root system properties on root system biomass of 10 North American grassland species

Dependence of the properties of root systems on the size of the root system may alter conclusions about differences in plant growth in different environments and among species. To determine whether important root system properties changed as root systems aged and accumulated biomass, we measured three important properties of fine roots (tissue density, diameter, and C:N) and three biomass ratios (root:shoot, fine:coarse, and shallow:deep) of monocultures of 10 North American grassland species five times during their second and third years of growth. With increasing belowground biomass, root tissue density increased and diameter decreased. This may reflect cortical loss associated with the aging of roots. For non-legumes, fine root C:N decreased with increasing root biomass, associated with decreases in soil solution NO₃^- concentrations. No changes in fine root C:N were detected with increasing belowground biomass for the two legumes we studied. Among all 10 species, there were generally no changes in the relative amounts of biomass in coarse and fine roots, root:shoot, or the depth placement of fine roots in the soil profile as belowground biomass increased. Though further research is needed to separate the influence of root system size, age of the roots, and changes in nutrient availability, these factors will need to be considered when comparing root functional traits among species and treatments.