| The deposition of inorganic N has increased several-fold in many regions due to human activities. Increased N supply can have a fertilization effect on forest ecosystems, but in the long-term, excess N can negatively impact biogeochemical cycling, soil chemistry, and productivity. Most research to date has focused on aggregate effects of N deposition on soil chemistry and N cycling. However, very little is known about the response of the microbial communities that are responsible for soil nutrient cycling and decomposition to these environmental changes. This dissertation addresses the effects of increased N on denitrification, which has not been well studied, and on microbial community structure, which may be a key regulator of ecosystem processes.; The effects of chronic N amendments and dolomite additions on N 2O and N2 gas emissions from denitrification were assessed at the Long-Term Soil Productivity (LTSP) experiment established with the Fernow Experimental forest in West Virginia. There were no detectable treatment effects on N2O or N2 flux from soils, though there was strong temporal variation. Denitrification enzyme activity was elevated in fertilized plots on some sample dates. These results suggest that whole-tree harvesting has altered the N status of these soils so that they are less prone to N saturation than more mature forests.; The response of microbial communities to N fertilization was evaluated in a cross-site study of three long-term experiments located in New England. In the two sites that appear to be N-saturated, microbial biomass strongly decreased in fertilized plots, and the ratio of fungi bacteria decreased. Using novel molecular techniques, the abundance of genes coding for enzymes involved in soil N cycling were quantified in these soils. The abundance of AmoA, associated with ammonia oxidation, strongly increased in fertilized plots at Harvard Forest. The abundance of denitrification genes either declined or was not affected by N fertilization at these sites. The differential response of these organisms can be attributed to differences in their physiology. Overall, these results suggest that soil microbes respond strongly to increased N in a manner that is likely to affect ecosystem processes. |
