Nitrogen-use-efficiency in Sphagnum mosses: Effects of atmospheric nitrogen deposition.

摘要

Sphagnum bogs are nutrient-poor ecosystems where plant growth is partly constrained by low nutrient levels. In these bogs, the entire water and nutrient supply is from atmospheric sources, such as precipitation, thus they are susceptible to changes in atmospheric chemistry. As atmospheric deposition of nitrogen increases from anthropogenic activity, it is likely that the Sphagnum mosses that dominate these bogs will be impacted directly. In this thesis, first I tested the hypothesis that Sphagnum nitrogen-use-efficiency decreases as nitrogen inputs from atmospheric deposition increase. I identified nitrogen retention from precipitation and internal nitrogen translocation as two key mechanisms for maximizing nitrogen-use-efficiency in Sphagna. I compared Sphagnum mosses in a high nitrogen deposition region in New York, to a low deposition region in Maine. I applied an 15N tracer to Sphagna in both regions, and estimated nitrogen retention and translocation.; Nitrogen retention ranged from 50% to 90%, and translocation ranged from 11% to 80% of nitrogen applied. Contrary to my hypothesis, both nitrogen retention and translocation appeared to be controlled more by climate and position of the water table rather than historical and recent nitrogen loads from atmospheric deposition. Further, these data suggest that nitrogen retention and translocation play equal roles in the Sphagnum nitrogen budget, each contributing approximately 10% of required nitrogen. Mineralization is likely an important sources to support Sphagnum nitrogen requirements.; Second, I proposed and tested a conceptual model of Sphagnum responses to increased atmospheric nitrogen deposition. The model included changes in tissue nitrogen concentrations and net primary production (NPP) as key parameters, and I tested it using my own data and data collected from the literature. I found three phases of Sphagnum responses to atmospheric nitrogen deposition, with critical nitrogen loads for each phase. Tissue nitrogen concentrations increased linearly with nitrogen deposition in the first phase, leveled off in the second phase, and ultimately decreased in the third phase. At all levels of nitrogen deposition, NPP was more strongly predicted by climatic variables than by nitrogen deposition. Results from this thesis indicate significant impacts of nitrogen loading on the Sphagnum mosses, even at low levels of nitrogen deposition.