Intensity and duration of exposure determine prokaryotic community response to salinization in freshwater wetland soils

• Published in: Geoderma 2022-12, Vol.428, p.116138, Article 116138
• Main Authors: Morina, Joseph C., Franklin, Rima B.
• Format: Article
• Language: English
• Subjects: Denitrification; DNRA; Microbial community; Nitrate reduction; Salinity; Saltwater intrusion
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2022.116138

•Salinization altered prokaryotic community composition and functional potential.•Response of community was predicated on salinity level and exposure length.•DNRA and denitrification rates affected differently by salinization.•Distinct profiles of nitrate reducers were observed in transplant communities.•Predicted metagenome data correlated with rates better than qPCR data. The salinization of coastal freshwater wetlands may impact the biogeochemical cycling of critical nutrients by altering soil microbial community structure and function. Nitrate (NO3–) reduction pathways appear particularly sensitive, though the interactive effects of salinization intensity and duration of exposure remain unclear. To address this knowledge gap, we performed a transplant experiment that exposed soil from a freshwater (≤0.1 ppt) wetland to oligohaline (1 ppt) and mesohaline (14 ppt) conditions for two years. In transplanted soils, we found physicochemical changes characteristic of salinization events, including elevated porewater concentrations of ammonium and sulfate and decreased soil organic matter and redox potential. Amplicon sequencing (16S rRNA) revealed that mesohaline levels of salinization caused rapid community restructuring; distinct transition communities were evident by the first sampling event (5 months) and persisted for ∼2 years (19–22 months). In contrast, freshwater communities transplanted to oligohaline conditions were highly resistant to restructuring, and it took nearly 2 years of salinization for differences to manifest. For both transplants, community shifts included changes in the distribution and abundance of taxa capable of NO3– reduction, including several groups also known for sulfur redox metabolism. Nitrate (15NO3-) reduction assays were performed to determine how rates were affected. Dissimilatory nitrate reduction to ammonium (DNRA) increased for all sampling events, but only under mesohaline conditions, whereas denitrification responses depended more on the duration of exposure than salinity level. These findings may be useful for determining when and how the ability of wetlands to remove nitrogen will be impacted by sea-level rise. Further, these results suggest that efforts to synthesize and generalize prior research regarding salinization effects on wetland microbial community structure and function must explicitly consider both salinity intensity and exposure length.