Influence of Plant Functional Group Removal on Inorganic Soil Nitrogen Concentrations in Native Grasslands

Abstract

High plant functional group diversity has been hypothesized to reduce resource concentrations based on the assumption that species from one functional group acquire resources similarly to one another, while species from other functional groups acquire resources dissimilarly. To determine if functional groups use soil nutrients different from one another, we investigated the impact of removing individual functional groups on soil inorganic nitrogen (NO-3 and NH+4 ) concentrations in the Idaho fescue (Festuca idahoensis Elmer)/bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve) habitat type in Montana. Treatments were imposed by removing 1) all plant species (total plant removal), 2) shallow-rooted (< 15 cm) forbs, 3) deep-rooted (> 15 cm) forbs, 4) all forbs (total forb removal), 5) grasses, and 6) spikemoss, compared to intact control plots. Inorganic nitrogen was measured at 2 soil depths (0-15 cm and 16-40 cm) in the spring, summer, and fall 1 year after treatment imposition. The removal of individual functional groups generally increased soil NO-3 and NH+4 concentrations. Total plant removal increased NO-3 concentrations more than removing individual functional groups. Grass removal generally increased soil NO-3 concentrations in the 0-15-cm depth more than other functional groups removal. Whether the grass or total forb removal treatment resulted in greater soil NH+4 concentrations in the 0-15-cm depth depended on season. These results suggest that functional groups vary in their soil nutrient acquisition patterns and that increased functional diversity decreases soil nutrient concentrations. Therefore, maintaining or improving functional diversity may be a method to more fully utilize soil nutrients because functional groups can differ in their spatial and temporal acquisition of resources.