Riggs, Charlotte2015-11-062015-11-062015-07https://hdl.handle.net/11299/175314University of Minnesota Ph.D. dissertation. July 2015. Major: Ecology, Evolution and Behavior. Advisor: Sarah Hobbie. 1 computer file (PDF); vi, 186 pages.Human activities have increased the availability of nutrients, such as nitrogen (N), phosphorus (P), and potassium (K), worldwide. Since alterations to nutrient cycles influence carbon (C) fixation and decomposition processes, nutrient enrichment affects global C stocks -- such as soil C. Carbon in soil organic matter (SOM) far outweighs vegetative C in the majority of biomes, especially grasslands. Consequently, either positive or negative changes to grassland soil C sequestration could feed back to influence the global C cycle. Unfortunately, the effects of nutrient enrichment on SOM cycling remain uncertain. In my dissertation, I examined the effects of nutrient addition on SOM cycling at participatory sites of the Nutrient Network -- a coordinated, global network of nutrient addition experiments that follow standard protocols for sampling and analysis. I found that the total soil C stock at experimental grasslands worldwide increased in response to the addition of N, P, and K. Furthermore, in a regional study in the US Central Great Plains, I found that N addition decreased microbial decomposition of SOM and tended to increase soil aggregation. Finally, in a laboratory study I found that decreased microbial biomass likely explains the decreased microbial decomposition of SOM in response to N addition. Overall, my results suggest that nutrient enrichment will lead to increased sequestration of soil C in some grassland soils.encarbondecompositiongrasslandmicrobenitrogensoilThesis or Dissertation