Nitrogen Fertilization Has a Stronger Effect on Soil Nitrogen-Fixing Bacterial Communities than Elevated Atmospheric CO2

Abstract

ABSTRACTBiological nitrogen fixation is the primary supply of N to most ecosystems, yet there is considerable uncertainty about how N-fixing bacteria will respond to global change factors such as increasing atmospheric CO2and N deposition. Using thenifHgene as a molecular marker, we studied how the community structure of N-fixing soil bacteria from temperate pine, aspen, and sweet gum stands and a brackish tidal marsh responded to multiyear elevated CO2conditions. We also examined how N availability, specifically, N fertilization, interacted with elevated CO2to affect these communities in the temperate pine forest. Based on data from Sanger sequencing and quantitative PCR, the soilnifHcomposition in the three forest systems was dominated by species in theGeobacteraceaeand, to a lesser extent,Alphaproteobacteria. The N-fixing-bacterial-community structure was subtly altered after 10 or more years of elevated atmospheric CO2, and the observed shifts differed in each biome. In the pine forest, N fertilization had a stronger effect onnifHcommunity structure than elevated CO2and suppressed the diversity and abundance of N-fixing bacteria under elevated atmospheric CO2conditions. These results indicate that N-fixing bacteria have complex, interacting responses that will be important for understanding ecosystem productivity in a changing climate.