The variable effects of soil nitrogen availability and insect herbivory on aboveground and belowground plant biomass in an old-field ecosystem
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Nutrient availability and herbivory can regulate primary production in ecosystems, but little is known about how, or whether, they may interact with one another. Here, we investigate how nitrogen availability and insect herbivory interact to alter aboveground and belowground plant community biomass in an old-field ecosystem. In 2004, we established 36 experimental plots in which we manipulated soil nitrogen (N) availability and insect abundance in a completely randomized plot design. In 2009, after 6 years of treatments, we measured aboveground biomass and assessed root production at peak growth. Overall, we found a significant effect of reduced soil N availability on aboveground biomass and belowground plant biomass production. Specifically, responses of aboveground and belowground community biomass to nutrients were driven by reductions in soil N, but not additions, indicating that soil N may not be limiting primary production in this ecosystem. Insects reduced the aboveground biomass of subdominant plant species and decreased coarse root production. We found no statistical interactions between N availability and insect herbivory for any response variable. Overall, the results of 6 years of nutrient manipulations and insect removals suggest strong bottom-up influences on total plant community productivity but more subtle effects of insect herbivores on aspects of aboveground and belowground production.
KeywordsCommunity structure Insect herbivory Old-field ecosystems Soil nitrogen Top-down
We thank Phillip Allen, Jacqueline Areson, Lauren Breza, Claire Brown, Windy A. Bunn, Karianne Chung, Melissa Cregger, Cavenne Engle, Colleen Iversen, Jean-Philippe Lessard, Laura Marsh, Martin Nuñez, Mariano Rodriguez-Cabal, Noelia Barrios, Tara Sackett, Tander Simberloff, Haley Smith, Katie Stuble, Heather Tran, Jeramy Webb, Jake Weltzin, and Phoebe Wright for help with field and laboratory work. Jake Weltzin was integral in establishing the experiment. The experiments described here complied with the current laws of the US. The work was partially supported by grants from Department of Ecology and Evolutionary Department at the University of Tennessee to J.B.
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