Ecology, structure, and function of denitrification and DNRA microbes in tidal freshwater wetlands

Background/Question/Methods

 Wetlands have the ability to decrease nutrient loads, preventing downstream eutrophication and coastal hypoxia, making these ecosystems targets for conservation and restoration. However, the microbial ecology that underlies this valuable ecosystem service is still understudied. Of particular concern is nitrogen cycling, and the microbial processes of denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA), which reduce NO₃^- to N₂ or NH₄⁺ respectively. The current study used organic matter (OM) and nitrate amendments to investigate the effects of resource availability on the abundance and activity of DNF- and DNRA-capable microorganisms in freshwater wetland soils. A modified litter bag design was used to augment wetland sediment with OM of variable lability (compost or leaf litter) and/or slow release nitrate (KNO₃) fertilizer in a full factorial design. Samples were incubated in situ for 3-12 months, and collected on a seasonal basis along with naive field samples. Upon recovery, samples were analyzed for functional group abundance (qPCR), and activity (anoxic slurry assay utilizing ¹⁵N).

Results/Conclusions

Denitrifiers were the dominant functional group under most conditions, but DNRA-capable organisms were present in appreciable numbers, and both groups were responsive to treatments. Denitrifiers had highest abundance in the presence of high quality OM (compost), while the abundance of DNRA organisms was enhanced by the addition of more recalcitrant material (leaf litter; two-way ANOVA, all p < 0.05). Activity rates were also interactively influenced by treatment, for instance, DNF rate showed a significant interaction between OM type and nitrate addition wherein highest rates were observed in the presence of nitrate and compost fertilization. Multivariate analysis revealed significant relationships between environmental factors, microbial community abundance, and activity rates; creating an integrated picture of the relationships between microbial ecology and biogeochemistry. Understanding the regulation of DNF and DNRA is important as the balance determines whether nitrate is retained (DNRA), removed (DNF), or unaffected by wetland ecosystems.