Stoichiometric responses to a suite of global change experiments in a mesic grassland

Background/Question/Methods

Global changes to the environment such as increased drought intensity and severity, altered inter- and intra-annual precipitation variability, and nitrogen deposition have the potential to affect ecosystem function directly as well as indirectly through changes in plant nutrient ratios and soil nutrient availability. Understanding the causes and consequences of these stoichiometric responses of species and communities may provide important mechanistic insight for future ecosystem function.  Twelve plant species spanning multiple functional groups, and the soils beneath them, were sampled for nitrogen and phosphorus concentrations across a gradient of nitrogen and phosphorus soil conditions resulting from a nine year nutrient addition experiment at the Konza Prairie Biological Station, Manhattan, KS, to obtain homeostasis values (defined as an organism’s ability to regulate internal stoichiometry despite variations in environmental stoichiometry) for each species. We then sampled three dominant tallgrass prairie species along with the soil beneath them for nitrogen and phosphorus concentrations within four existing experiments administering the following treatments: two years of intense drought (-66% growing season precipitation), twenty-one years of water addition (+31% of ambient precipitation), fifteen years of increased variability of rainfall amount, and four years of nitrogen and phosphorus addition (10 g m^-2yr^-1) to: 1) Determine the relationships between dominance/stability and stoichiometric homeostasis in North American tallgrass prairie; 2) Formulate and test predictions for stoichiometric responses of dominant species to climate change; 3) Determine the effects of changing soil nutrient concentrations on species abundance and ecosystem stability.

Results/Conclusions

Under ambient climatic conditions, a strong positive relationship was determined between a species’ stoichiometric homeostasis and its dominance and stability. We then used the derived homeostatic models to make predictions of leaf-level stoichiometric responses to changes in soil nutrients brought about by the four experiments mentioned above. These predictions were assessed with leaf level stoichiometric measurements taken in 2011. These homeostatic models explained 94% of the variation in leaf nitrogen, 65% of the variation in leaf phosphorus, and 31% of the variation in leaf N:P among the three species. We also used the homeostatic models to assess species cover shifts in a nitrogen addition experiment conducted from 2002 to 2011. We found a negative relationship between species cover response to nitrogen addition with stoichiometric homeostasis explaining 24% of the variation. Ultimately, stoichiometric homeostasis was found to be a useful metric to predict stoichiometric, compositional, and functional responses of species and ecosystems to many aspects of global change.