Master of Environmental Science, Miami University, 2017, Environmental Sciences

The health of forest ecosystems depend on the recycling of key nutrients, especially nitrogen (N) (Bormann et al. 1977). A decline in N mineralization from high to low elevation at the Hubbard Brook Experimental Forest has been attributed to less snowpack at low elevations and the consequent disruption of its insulating effects on microbial activity (Campbell et al. 2009). A potential mechanism by which more frequent soil freezing causes lower ecosystem N recycling is more rapid movement of N from surface horizons, where N tends to be strongly retained, into mineral soils where there is greater potential for loss. This project investigates climate effects on N cycling using an elevation gradient in northeastern hardwoods at the HBEF. Movement of labile organic N from a single-year's litterfall was sensitive to winter climate for the first two winter seasons. Potentially-mineralizable 15N was higher in mineral soils in low elevations in the first growing season, but was higher in the Oe and mineral soil of high elevation sites in the second growing season. However, inorganic 15N in the mineral soil was markedly greater in high- than low-elevation sites during the first growing season. These results suggest more efficient retention of litter-derived N at higher elevation, but do not support the hypothesis that less winter snow cover promotes losses of litter derived N from mineral soil.

Committee: Melany Fisk (Advisor); Jonathan Levy (Committee Member); Tom Crist (Committee Member) Subjects: Biology; Climate Change; Ecology; Environmental Science; Soil Sciences

Doctor of Philosophy, The Ohio State University, 2007, Evolution, Ecology, and Organismal Biology

Nitrogen (N) is the nutrient most limiting to plant growth (NPP) in temperate forests. In N-limited temperate forests, most of the N required for NPP is recycled between soil and plant N pools by the microbial process of N-mineralization (Nmin). However, human activities have increased atmospheric N deposition (Ndep) to forests in the last 50-100 years, and this surplus N may increase NPP. But, forest responses to Ndep are not satisfactorily understood, and depend on how atmospheric N inputs are partitioned between soils and plants. From my field data collection at a mature forest site, I estimated that NPP required 51 kg N ha-1 yr-1, most of which was used for fine root and leaf production (62% and 31%, respectively). Each year, Nmin supplied 87% of Nreq, and Ndep contributed an additional 13%, 4% of which was due to canopy retention of Ndep (Ncr). Data from my mesocosm 15N-labelling experiment suggested that very little (<10%) of Ncr observed in the field was actually taken up by trees, and the majority of Ndep (>85%) was assimilated into soil pools. These results suggest that Ndep could not have significantly increased forest NPP at UMBS over the time scale of my studies. My greenhouse experiment corroborated this conclusion, with tree seedlings showing no significant increase in photosynthesis or growth in response to Ndep at ambient rates. However, Ndep to forest ecosystems has been occurring for decades 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 (open full item for complete abstract)

Committee: Peter Curtis (Advisor) Subjects: Biology, Ecology

Master of Science, Miami University, 2009, Zoology

Anthropogenic soil acidification appears to be detrimental to forest health in the northeastern US possibly due to reduced microbial activity influencing plant-available nutrients. In a recent study at the Hubbard Brook Experimental Forest, soil microbial activity was not stimulated by increased pH. Therefore, microbial response to increased pH may depend on the level of calcium added or availability of other nutrients. I tested the effects of Ca and phosphorus additions in the field on microbial nitrogen transformations. High Ca addition reduced net N mineralization. Low Ca addition did not affect N transformations, but the combination of Ca/P addition reduced net N mineralization. Phosphorus addition unexpectedly increased gross nitrification. Results from this study indicate that microbial mineralization is not sensitive to moderate differences in pH or limited by P. Therefore, detrimental effects of acidic deposition to these forests are not likely a result of nutrient deficiencies related to suppression of microbial activity.

Committee: Melany Fisk C (Advisor); Michael Vanni J (Committee Member); Thomas Crist O (Committee Member) Subjects: Biogeochemistry; Biology; Ecology; Soil Sciences