Experimental Nitrogen Disposition Influences Microbial Denitrifying Communities and Increases Denitrification Rates in a Northern Hardwood Forest

Abstract

With more biologically reactive nitrogen (N) becoming available in terrestrial ecosystems due to atmospheric N deposition, it is important to understand its impact on forest biogeochemistry. Long-term experimental N deposition in a northern hardwood ecosystem increased the assimilation of N by plants (Zak et al., 2004) as well as dramatically increased the export of NO3- via leaching (Pregitzer et al., 2004). This study aimed to quantify changes in denitrification, another possible fate of N, and changes in the bacterial denitrifier community after more than 16 years of experimental N deposition. We hypothesized that the increased NO3- availability would increase denitrification rates and N2O production due to an increase in gene abundance and changes in composition of bacterial denitrifiers. Using laboratory assays, we found that denitrification rates and N2O production were significantly higher under experimental N deposition. However, contrary to our hypothesis, five denitrification genes (norB, narG, nirK, nirS, and nosZ) decreased under experimental N deposition. Additionally, no relationship occurred between the abundance of nosZ and N2O production. This apparent uncoupling of functional gene abundance and function may be due to differences in denitrifier community membership. These results indicate that the observed increases in denitrification rates and N2O production, shift in denitrifier composition, and decrease in functional gene abundance demonstrate that chronic atmospheric N deposition has altered denitrification on both a molecular and ecosystem-level scale, albeit these responses are small at an ecosystem level.