Eutrophication amplifies foliar fungi and insect trophic controls on grassland carbon cycling

Max Zaret, Eric W. Seabloom and Elizabeth T. Borer, Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN, Linda L. Kinkel, Plant Pathology, University of Minnesota, Saint Paul, MN

Background/Question/Methods
Eutrophication and changes in food web composition are predicted to interactively impact terrestrial ecosystem carbon fluxes. However, experimental evidence of plant consumer and nutrient controls has largely focused only on herbivores while ignoring the role of foliar or soil fungi as potential mediators of ecosystem carbon fluxes. Here we measured the effects of nutrient limitation and distinct consumer groups on instantaneous rates of ecosystem carbon fluxes within a grassland field experiment in East Bethel, Minnesota. We annually applied nitrogen, phosphorus, and potassium as well as experimentally removed foliar fungi, soil fungi, arthropods, large mammalian herbivores, or all 4 trophic groups in experimental plots using regular applications of pesticides or fencing. To assess the relative impacts of nutrient limitation and consumer groups on gross primary production (GPP), ecosystem respiration (ER), and their difference, net ecosystem exchange (NEE), we measured changes in CO2 concentration in experimental plots using an infrared gas analyzer. Using these data, we evaluate the interactive effects of nutrients and consumers on rates of grassland carbon fluxes and whether impacts vary among consumer groups.
Results/Conclusions
Nutrient addition significantly increased GPP by 15% compared to unfertilized plots, and the combination consumer removal treatment increased GPP by 36% relative to plots with no consumer removal treatment. In addition, nutrient addition interacted with consumer removal on GPP, with foliar fungi and arthropod removal having no impact in unfertilized plots but increasing GPP in fertilized plots (by 29% and 26%, respectively). This suggests that nutrient additions can potentially amplify consumer controls by both microbes and herbivores. In contrast to GPP, ER was impacted by consumers but not nutrients, with an ER increase of 25% in the combination consumer removal treatment. This resulted in nutrient addition increasing NEE by 45% due to the significant impacts on GPP and no impacts on ER, while consumers had no significant impacts on NEE due to the similar impacts on both GPP and ER. Taken together, these results provide new perspectives on the joint roles of nutrients and consumers in regulating ecosystem carbon fluxes in grasslands. Our work reveals that plant consumers, both microbial and animal, mediate rates of ecosystem carbon fluxes and such impacts may be dependent on the nutrient supply of the ecosystem. These findings suggest that eutrophication caused by human activities may modify consumer impacts on plant productivity and carbon cycling, calling for further integration of food webs and top-down controls in ecosystem carbon models.