Long-term fertilization modifies the composition of slow-growing oligotrophs more than fast-growing copiotrophs in a nutrient poor coastal plain wetland

Abstract

Wetland ecosystems are known for their carbon storage potential due to slow decomposition rates and high carbon fixation rates. However, nutrient addition from human activities affects this carbon storage capacity as the balance of fixed and respired carbon shifts due to plant and microbial communities. Ongoing atmospheric deposition of nutrients could be changing wetland plant-microbe interactions in ways that tip the balance from carbon storage to loss. Therefore, examining microbial community patterns in response to nutrient enrichment is important to understanding nutrient effects on carbon storage potential. In this study, we hypothesized that fertilization of a low nutrient ecosystem leads to increased organic carbon input from plant biomass into the soil and results in increased soil bacterial diversity and modifications to soil bacterial community composition. As such, increased soil nutrients and carbon resources provide more energy to support increased microbial growth rates, which can result in wetland carbon losses. To test this hypothesis, we used bacterial community-level and soil chemical data from the long-term wetland ecology experiment at East Carolina Universitys West Research Campus (established in 2003). Specifically, we examined the extent that long-term effects of nutrient enrichment affect wetland microbial communities and plant biomass, which are factors that can affect carbon storage. We collected soil cores from fertilized and unfertilized test plots. We extracted genomic DNA from soil samples and conducted 16S rRNA targeted amplicon sequencing to characterize the bacterial community composition. In addition, we measured plant above and belowground biomass and soil carbon content. Results revealed an increase in aboveground plant biomass, soil carbon, and bacterial diversity. In contrast, belowground plant biomass and microbial biomass were similar in fertilized and unfertilized plots. To further examine bacterial community changes to nutrient enrichment, we compared the relative abundance of fast growing copiotrophic and slow growing oligotrophic bacteria of a subset of taxa putatively identified as belonging to either life history strategy. These taxa-level results revealed a decrease in oligotroph relative abundance and little to no change in copiotroph relative abundance of a subset of bacterial taxa. If there is a community-wide shift in the proportion of oligotroph to copiotroph life history strategies, this would have a negative impact on organic carbon storage since oligotrophic bacteria respire less carbon than copiotrophic bacteria over the same amount of time. Taken together, this study provided evidence that long-term nutrient enrichment influences wetland soils in ways that decrease their carbon storage potential of important carbon sinks.