Nitrogen Fertilization Has a Stronger Effect on Soil Nitrogen-Fixing Bacterial Communities than Elevated Atmospheric CO ₂

Abstract

c Biological nitrogen fixation is the primary supply of N to most ecosystems, yet there is considerable uncertainty about how Nfixing bacteria will respond to global change factors such as increasing atmospheric CO 2 and N deposition. Using the nifH gene 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 CO 2 conditions. We also examined how N availability, specifically, N fertilization, interacted with elevated CO 2 to affect these communities in the temperate pine forest. Based on data from Sanger sequencing and quantitative PCR, the soil nifH composition in the three forest systems was dominated by species in the Geobacteraceae and, to a lesser extent, Alphaproteobacteria. The N-fixing-bacterial-community structure was subtly altered after 10 or more years of elevated atmospheric CO 2 , and the observed shifts differed in each biome. In the pine forest, N fertilization had a stronger effect on nifH community structure than elevated CO 2 and suppressed the diversity and abundance of N-fixing bacteria under elevated atmospheric CO 2 conditions. These results indicate that N-fixing bacteria have complex, interacting responses that will be important for understanding ecosystem productivity in a changing climate. N itrogen is the most common nutrient limiting productivity in terrestrial ecosystems and enters ecosystems predominantly through bacterial fixation. However, we lack a clear understanding of how N-fixing bacteria respond to climate change drivers or how conserved those responses might be across biomes in a geographic region. Nonagricultural biomes in the eastern United States that experience elevated atmospheric CO 2 and increasing N deposition include hardwood forests to the north, pine forests to the south, and brackish marsh areas along the eastern seaboard. Rising atmospheric CO 2 concentrations and shifting patterns of N deposition can interact and affect N fixation processes in soil (1). To determine if populations of N-fixing bacteria in soils of different biomes showed similarities in composition and in responses to elevated CO 2 , we conducted a systematic survey of soil N-fixing bacterial communities across four biomes in the eastern United States, utilizing long-term, free-air CO 2 enrichment (FACE) experiments (2). One of these field experiments combined elevated CO 2 and N fertilization treatments, allowing us to determine their interactive effects on the N-fixing community in a pine forest in the southeastern United States.Progressive N limitation theory proposes that ecosystems become more N limited with rising CO 2 , which suggests that the continued sequestration of CO 2 in terrestrial biomass will require greater N fixation inputs (3, 4). The increased ecosystem demand for N under elevated CO 2 has been documented after several years of whole-forest CO 2 enrichment (5, 6). N-fixing bacteria, which span many taxonomic groups with high levels of ...

The Diversity and Co-occurrence Patterns of N2-Fixing Communities in a CO2-Enriched Grassland Ecosystem

“…Different colors refer to different phyla ecosystem, the observation was consistent with several recent studies [7,53]. For example, Berthrong et al found relatively small effects of CO 2 treatment on N-fixing bacterial community in the four long-term eCO 2 experimental sites they investigated, and no consistent differences were observed for nifH diversity between aCO 2 and eCO 2 soils [7]. Notably, although only two OTUs were found for the genus Frankia, they were about eightfold more abundant than in aCO 2 , which is also consistent with a previous study that the activity of Frankia species increased with infertile soil in response to eCO 2 [54].…”

“…16S OTUs were represented by circular shape. Different colors refer to different phyla ecosystem, the observation was consistent with several recent studies [7,53]. For example, Berthrong et al found relatively small effects of CO 2 treatment on N-fixing bacterial community in the four long-term eCO 2 experimental sites they investigated, and no consistent differences were observed for nifH diversity between aCO 2 and eCO 2 soils [7].…”

“…Notably, although the PolF/PolR primer sets we used in this study were of relatively low coverage (∼25 %) among all available nifH primers [55], they were still selected for their higher specificity with much fewer bands after PCR amplification, even though a high diversity of nifH community was observed, with OTUs from six major phyla (ten classes), among which Proteobacteria (Alpha-and Beta-) are the most dominant groups, which is generally consistent with several previous studies in soil [7,8]. Although found with high diversity, 271 OTUs were still not classified to any taxonomic groups at 80 % identity cutoff, suggesting a highly diverse genetic pool for nifH genes.…”

“…Similar to that in ocean, it is believed that both symbiotic and free-living diazotrophs contribute significantly to the terrestrial N budget [15]. Although many previous studies focused on the relationship between symbiotic diazotrophs and plants, the diversity and community structure of diazotrophic communities were recently analyzed by several studies [7,8,[11][12][13]16]. It has also been pointed out that the N 2 fixation rates in soil were significantly affected by diazotrophic community structure [11].…”

“…During the past decades, the nifH gene family has been widely analyzed to study diazotrophic microbial communities in various environments, especially in marine and soil ecosystems [5][6][7][8][9][10][11][12][13], resulting in novel insights into the diversity and structure of N 2 -fixing communities. For example, even though Trichodesmium has been found as the major N 2 -fixing cyanobacteria in marine ecosystems [6], a recent study suggested that the contributions of other N 2 fixers are far more significant than previously estimated [14], indicating the important roles of less dominant N 2 fixers.…”