Over the last century, human activities have increased global nitrogen and phosphorus deposition rates. This nonpoint source nutrient enrichment can be particularly disruptive to historically low nutrient ecosystems. Fertilization effects are known to increase carbon cycling rates; nutrient feedbacks on plant-microbial interactions have the potential to influence long-term carbon storage. For example, whether a wetland is a carbon sink or source is driven, in part, by the rates of carbon fixation by plants and fungal and bacterial decomposition of soil organic matter. We can gain insights into mechanisms that determine carbon storage or loss by understanding how microbial metabolism changes in response to nutrient enrichment. Since 2003, a factorial experiment located in the eastern North Carolina coastal plain has been maintained to study the effects of nutrient addition (N-P-K fertilizer), disturbance (mowing), and their interaction on wetland plant and microbial community structure and function. To investigate the extent that nutrient enrichment influences fungal and bacterial community composition and microbial metabolic activity, we compared microbial community structure-function relationships in nutrient enriched vs. ambient plots.
Results/Conclusions
Results reveal that bacterial taxonomic diversity (16S rRNA amplicon sequencing) associated with both bulk and plant rhizosphere (grass vs. forb) soils are higher in fertilized compared to unfertilized soils. In addition, soil nutrient concentrations, particularly soil ammonium and phosphate, strongly determine bacterial and fungal community composition, while plant association also explains community composition but to a lesser degree. Nutrient enrichment also influenced microbial function; we observed higher metabolic rates and substrate use diversity (based on phenotypic microarray assays, litter decomposition) in fertilized compared to unfertilized conditions. Morphological characteristics of bacterial isolates were similar across fertilization treatments; however, we also observed higher morphological diversity in fertilized compared to unfertilized plots. This study suggests that microbial communities from fertilized plots have greater substrate use diversity and faster substrate use rates. Over time, nutrient enrichment of historically low nutrient ecosystems could alter carbon storage potential due to greater metabolic diversity of the microbial community. This work will identify substrates and microbial community members to target as control points of carbon cycling in wetlands.