|Title||Nitrogen cycling in the northen hardwood forest: soil, plant, and atmospheric processes|
|Year of Publication||2007|
|Degree||Doctor of Philosophy|
|Number of Pages||89 pp.|
|University||The Ohio State University|
Nitrogen (N) is as important to forest ecosystem processes as water, sunlight, and carbon (C), and is often the mineral nutrient most strongly limiting plant growth (NPP) and microbial activity. In N-limited temperate forests, most of the N required for tree growth is internally recycled between soil and plant N pools through decomposition of organic detritus, or N-mineralization (Nmin). However, within the last 50-100 years, humans have significantly altered the global N cycle, leading to increased atmospheric N inputs to forests. Fossil fuel combustion, fertilizer- and manure-intensive agriculture, and biomass burning all mobilize reactive N into the atmosphere, where it can be transported thousands of kilometers before being deposited to ecosystems. Decades of debate and research on N inputs to forest ecosystems have indicated that atmospheric N deposition (Ndep) may increase NPP. But, the temporal scales of forest process responses to Ndep are not satisfactorily understood, and depend on how atmospheric N inputs are partitioned between soils and plants. For my dissertation research, I studied soil, plant, and atmospheric aspects of the N cycle in a regionally representative northern hardwood forest. Field measurements of soil and atmospheric N supply processes, and the N requirement of NPP (N req) at the University of Michigan Biological Station (UMBS) formed the foundation for my work. I tested field observations of Ndep and forest canopy retention of N deposition (Ncr) in a mesocosm experiment, using 15N as a label to observe how Ndep and Ncr partitioned into plant and soil N pools. I also investigated the effects of this partitioning on tree seedling growth and physiology in a greenhouse study. The final element of my dissertation research was a meta-analysis of the effects of N addition on forest soil chemistry and N cycling. From my field data collection, I estimated that forest NPP from 1999-2005 required approximately 51 kg N ha-1 yr-1, most of which was used for fine root and leaf production (62% and 31%, respectively). On an annual basis, Nmin supplied 87% of Nreq, while Ndep contributed an additional 13%. Forest canopy retention of Ndep provided =4% of the forest+s annual NPP N requirement. Data from my 15N labelling experiment suggested that very little (<10%) of Ncr was actually incorporated into trees via foliar uptake, and that the majority of Ndep (>85%) was rapidly assimilated into soil N pools. These results suggested that Ndep could not have significantly increased forest NPP at UMBS over the time scale of my studies. My greenhouse experiment corroborated this conclusion, as there was no significant increase in photosynthesis or growth among tree seedlings exposed to Ndep at regional ambient rates. However, Ndep to forest ecosystems has been occurring for 50-100 years in industrialized regions, and most of the N inputs have been incorporated into soil organic matter (SOM). Research across temperate forests has suggested that forests exposed to large N inputs over time exhibit decreased soil C/N ratios, which are associated with faster Nmin rates. Using meta-analysis, I verified this pattern in the literature, and discovered novel relationships between forest soil properties and their responses to N inputs. My results demonstrated a long-term, quantitative relationship between Ndep and Nmin, and suggest that NPP may increase in temperate forests affected by Ndep.