PLANT TRAITS AND RESOURCE REDUCTION FOR 5 GRASSES GROWING ON A NITROGEN GRADIENT

Five grass species (Agrostis scabra, Agropyron repens, Poa pratensis, Schizachyrium scoparium and Andropogon gerardi) were grown in monoculture for 3 yr on an experimental nitrogen gradient. The species differed significantly in the levels to which they reduced soil solution (0.01 mol/L KCl extractable) nitrate and ammonium concentrations and light penetration to the soil surface. Soil nitrate concentration was an inverse function of root mass, which explained 73% of the observed variance in nitrate. Other species differences explained an additional 9.2%, and total soil N an additional 5% of this variance. Extractable soil ammonium also depended on these variables, but total soil N explained the most variance. Light penetration to the soil surface in these monocultures was a negative exponential function of aboveground biomass (R2 = 0.79). Schizachyrium and Andropogon, the species that reduced soil solution N to the lowest levels on infertile soils, had lower vegetative growth rates, higher root allocation, lower reproductive allocation, and lower tissue N than the other species. Many of these traits are associated with plants of infertile habitats, suggesting a direct link between ecophysiology, resource reduction, and distributional patterns. Because all species survived on even our most nitrogen-poor soil (subsurface sand), differential nutrient reduction, not tolerance, may be the main mechanism favoring these traits in infertile habitats. On infertile soils, the three earlier successional species (Agrostis, Agropyron, and Poa) allocated more to reproduction (rhizome or seed) than the later successional species, but did not reduce soil solution nitrate and ammonium to as low levels. This suggests that our early successional species may be superior colonists but inferior nitrogen competitors compared to the prairie bunchgrasses. Our results can be used to make explicit predictions as to the outcome of nitrogen competition among all possible combinations of these five species.