High rates of dormancy mask the effect of fertilization on microbial activity in salt marshes

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Salt marshes are highly productive ecosystems that provide numerous ecosystem services.

Despite their importance, salt marshes have experienced substantial declines in recent

decades, partly due to excess anthropogenic nitrogen additions. Salt marshes harbor

abundant, highly diverse, and active microbial communities, but we have a poor

understanding of how they respond to changes in their environment, in particular to nutrient

enrichment. To address this knowledge gap, we examined sediment microbial communities

from four creeks nested within a decade-long marsh fertilization experiment as part of the

TIDE project at the Plum Island Ecosystems LTER during 2005-2006 and 2013-2014. We

sampled sediment underlying the dominant macrophytes (tall Spartina alterniflora and

Spartina patens) monthly over the course of the growing season (May-October) and

sequenced both 16S rDNA and 16S rRNA to analyze the total (16S rDNA) and active (16S

rRNA) microbial communities. When examining the total microbial communities, we observed

no effect of fertilization, while a strong effect of habitat was evident. Analysis of 16S rRNA,

however, revealed a dramatic effect of fertilization on the active microbial communities. The

active communities in the unfertilized sediments were strongly structured by both season and

plant type, while no such patterns existed in the fertilized sediments suggesting a

homogenization of the active microbial communities. Additionally, active communities in the

fertilized sediments from 2013 and 2014 showed distinct compositions from corresponding

communities in 2005 and 2006, suggesting long-term effects of fertilization on active

microbes. Our results suggest fertilization elicits a homogenization effect on microbial

communities, overwhelming the effects of season and habitat observed in unfertilized

sediments and reducing overall microbial diversity.