Relationship between the structure of root systems and resource use for 11 North American grassland plants

Eleven Midwest North American grassland plant species differed in their construction, production, and placement of fine and coarse belowground biomass in the soil profile after having been grown in containers in the field for two and a half growing seasons. Based on the patterns of root system structure and resource utilization, the species we examined could be classified as 1) legumes, 2) high-nitrogen rhizomatous C-3 species, and 3) a separate gradient of differentiation from tall- to short-statured species (i.e. tallgrass to shortgrass species). Legumes depleted water evenly throughout the soil profile, with little capacity for acquisition of inorganic nitrogen throughout the 1m soil profile. The three rhizomatous species had shallow fine root distributions, a large relative investment in shallow rhizomes, and moisture and NO3- levels were low in shallow soils, but high at depth. Tallgrass species maintained a large standing root biomass of high-density, low-nitrogen fine roots, and acquire nitrogen and water from a large, deep volume of soil, in which inorganic nitrogen is present in low concentrations. Root systems of shortgrass species lacked coarse belowground biomass, had fine roots that were finer than those of the tallgrass species, and had a shallow root distribution. There was little support for functional dichotomies between the C-3 and C-4 species or between the grasses and forbs. For example, Solidago rigida (C-3 forb) and Andropogon gerardii (C-4 grass) were more similar to each other than to other C-3 forbs or C-4 grasses, respectively. Across all species and depths examined, there were strong relationships between the amount of fine root biomass present in a unit of volume of soil and the depletion of soil water and nitrogen, but there were no relationships with coarse belowground biomass. This reaffirms that differentiation of coarse and fine root biomass is as important as differentiating stems and leaves in evaluating plant allocation and ecosystem functioning.