The Effect of Long Term Nitrogen Deposition on the Structure and Function of Soil Microbial Communities.

Microbes (bacteria and fungi) drive biogeochemical cycling in soil by transforming nutrients through processes such as decomposition and nitrogen fixation. Nutrient transformation rates are limited by both the biomass of the microbial community and the functional capability of the community members. Therefore, when the microbial community is altered by ecosystem disturbance, there may be subsequent impacts on biogeochemical processes. Atmospheric nitrogen (N) deposition is a source of chronic N enrichment that impacts large areas including forested watersheds which may be otherwise relatively unaffected by anthropogenic activity. Previous research has shown that N deposition can result in higher N concentrations in the forest floor and increased export to comparatively sensitive aquatic systems. Furthermore, prolonged N deposition can alter microbial community composition by reducing fungal biomass. The objective of this study is to determine how long term atmospheric N deposition has affected soil microbial communities and by extension, ecosystem function. Microbial community composition and biomass were measured via fatty acid methyl ester (FAME) analysis on soils collected from the Bear Brook Watershed in Maine (BBWM), a paired watershed experiment where one of two contiguous watersheds has been subjected to bimonthly ammonium sulfate additions since 1989. Our results show that the fungal to bacterial biomass ratio decreased nearly 30% in organic soils and 20% in mineral soils as a result of N additions and overall community composition is affected by soil and vegetation type in addition to N additions. We will further assess the effect of N deposition on microbial function through quantitative PCR (qPCR) of key N-cycling genes.