Effects of Increased Nitrogen and Phosphorus Availability on Plant Productivity and Nutrient Use at Multiple Ecological Scales in Northern Peatlands

The adaptive use of resources by plants is an important topic in ecology, and is generally expressed as a resource-use efficiency. Nitrogen-use efficiency (NUE), in particular, has been the subject of several studies, as nitrogen (N) is the primary growth-limiting factors in many terrestrial systems. in chapter three, we examined how anthropogenic increases in N and phosphorus (P) availability may affect plant NUE response at multiple ecological scales due to carbon and N allocation within the entire plant (the leaf, woody tissue, above- and belowground, and whole-plant) and changes in species composition within the community. We examined plant nutrient efficiency indices in fertilization experiments (6 g N m-2 yr-1, 2 g P m-2 yr-1, or a combination of N and P) in nutrient-limited peatland ecosystems in the Upper Peninsula of Michigan. We further examined the evolutionary tradeoff of parameters such as N-productivity and the mean residence time (MRT) of N in biomass, and subsequent effects on NUE. Lastly, we examined the plant community response to environmental nutrient availability and N-uptake efficiency (plant N uptake/soil N availability). We found that N and P fertilization generally increased aboveground net primary productivity (ANPP) and tissue N concentration, although NUE response to nutrient addition was not straightforward. NUE differed by plant species, and across the ombrotrophic-minerotrophic gradient, and was often affected by the evolutionary tradeoff between N-productivity and MRT, where plants and communities were phenotypically and genetically adapted to maximize either N-productivity or MRT, but not both concurrently. However, N and P fertilization ubiquitously affected plant community N-uptake efficiency, and ultimately N-response efficiency, though the response to each nutrient was dependent on the plant species and community examined. Thus, plant community response to soil N availability may exert more important ecosystem controls than NUE, as significant changes in N-uptake efficiency at various ecological scales may affect ecosystem carbon and nutrient cycling processes.