Abstract
Salt marshes provide many key ecosystem services that have tremendous ecological and economic value. One critical service is the removal of fixed nitrogen from coastal waters, which limits the negative effects of eutrophication resulting from increased nutrient supply. Nutrient enrichment of salt marsh sediments results in higher rates of nitrogen cycling and, commonly, a concurrent increase in the flux of nitrous oxide, an important greenhouse gas. Little is known, however, regarding controls on the microbial communities that contribute to nitrous oxide fluxes in marsh sediments. To address this disconnect, we generated profiles of microbial communities and communities of micro-organisms containing specific nitrogen cycling genes that encode several enzymes (amoA, norB, nosZ) related to nitrous oxide flux from salt marsh sediments. We hypothesized that communities of microbes responsible for nitrogen transformations will be structured by nitrogen availability. Taxa that respond positively to high nitrogen inputs may be responsible for the elevated rates of nitrogen cycling processes measured in fertilized sediments. Our data show that, with the exception of ammonia-oxidizing archaea, the community composition of organisms involved in the production and consumption of nitrous oxide was altered under nutrient enrichment. These results suggest that previously measured rates of nitrous oxide production and consumption are likely the result of changes in community structure, not simply changes in microbial activity.
Highlights
Salt marshes provide numerous ecosystem services (Deegan, 1993; Chmura et al, 2003; Gedan et al, 2011) including the removal of fixed nitrogen (N) from the environment (Valiela and Teal, 1979)
These steps are mediated by the enzymes nitric oxide reductase (NOR) and nitrous oxide reductase (N2OR), which are frequently assessed in the environment by examination of the norB and nosZ genes, respectively (Dalsgaard et al, 2014; Kearns et al, 2015)
Moisture content varied as a function of treatment (ANOVA, F = 39.57, p < 0.001; Figure 1C) as XF plots were significantly drier than HF (Tukey HSD, p < 0.001) and C (p < 0.001) plots
Summary
Salt marshes provide numerous ecosystem services (Deegan, 1993; Chmura et al, 2003; Gedan et al, 2011) including the removal of fixed nitrogen (N) from the environment (Valiela and Teal, 1979). Salt marsh sediments are hotspots for other processes in the N cycle (Kaplan et al, 1979; Valiela and Teal, 1979). Nitrification supplies the NO3− that is needed for denitrification, which is typically limiting in oxygen-depleted sediments This linkage, referred to as coupled nitrification– denitrification, often represents the largest N-loss process from salt marshes (Patrick and Reddy, 1976; White and Howes, 1994). Two clades of bacteria contain nosZ genes that are phylogenetically distinct from the canonical denitrifier nosZ, termed atypical nosZ These organisms can scavenge free N2O from the environment, possibly making them important N2O sinks in salt marsh sediments (Sanford et al, 2012; Jones et al, 2013). As has been seen in this and other marsh fertilization experiments (Bowen et al, 2011; Kearns et al, 2016), we hypothesized that the overall microbial community, which includes both active and dormant microbes, would not vary as a function of fertilization, but the potentially active community would show a shift in community structure as nutrient enrichment increases
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