Coastal marshes are important blue carbon reservoirs, but it is unclear how vegetation shifts associated with tidal restoration and sea level rise alter soil microbial respiration rates and bacterial community composition. Within 20 Connecticut salt marshes (10 without tidal restrictions, 10 tidally restored), we sampled three vegetation zones dominated by Spartina alterniflora (short-form, < 30 cm tall), S. patens, and Phragmites australis to estimate microbial respiration rates (SIR, substrate-induced respiration; carbon mineralization), root zone bacterial 16S rRNA genes, and a suite of plant and soil characteristics. Carbon density was greater in unrestricted marshes than tidally restored marshes and was the only parameter that differed among sites with varying restoration histories. We observed strong differences among vegetation zones, with vegetation being a top predictor of both SIR and carbon mineralization. Electrical conductivity (EC) was also a top predictor for SIR, and we observed strong, positive correlations between EC and both metrics of microbial respiration, with elevated rates in more frequently inundated S. alterniflora than P. australis zones. We also observed distinct root zone microbial communities associated with vegetation zones, with greater abundance of sulfate-reducing bacteria in Spartina spp. zones. Our findings suggest that dominant salt marsh vegetation zones are useful indicators of hydrologic conditions and could be used to estimate microbial respiration rates; however, it is still unclear whether differences in microbial respiration and community composition among vegetation zones are driven by plant community, environmental conditions, or their interactions.

Salt marsh vegetation zones shift in response to large-scale environmental changes such as sea-level rise (SLR) and restoration activities, but it is unclear if they are good indicators of soil nitrogen removal. Our goal was to characterize the relationship between denitrification potential and salt marsh vegetation zones in tidally restored and tidally unrestricted coastal marshes, and to use vegetation zones to extrapolate how SLR may influence high marsh denitrification at the landscape scale. We conducted denitrification enzyme activity assays on sediment collected from three vegetation zones expected to shift in distribution due to SLR and tidal flow restoration across 20 salt marshes in Connecticut, USA (n = 60 sampling plots) during the summer of 2017. We found lower denitrification potential in short-form Spartina alterniflora zones (mean, 95% CI: 4, 3-6 mg N h À1 m À2 ) than in S. patens (25, 15-36 mg N h À1 m À2 ) and Phragmites australis (56, 16-96 mg N h À1 m À2 ) zones. Vegetation zone was the single best predictor and explained 52% of the variation in denitrification potential; incorporating restoration status and soil characteristics (soil salinity, moisture, and ammonium) did not improve model fit.Because denitrification potential did not differ between tidally restored and unrestricted marshes, we suggest landscape-scale changes in denitrification after tidal restoration are likely to be associated with shifts in vegetation, rather than differences driven by restoration status. Sea-level-rise-induced hydrologic changes are widely observed to shift high marsh dominated by S. patens to short-form S. alterniflora. To explore the implications of this shift in dominant high marsh vegetation, we paired our measured mean denitrification potential rates with projections of high marsh loss from SLR. We found that, under low and medium SLR scenarios, predicted losses of denitrification potential due to replacement of S. patens by short-form S. alterniflora were substantially larger than losses due to reduced high marsh land area alone. Our results suggest that changes in vegetation zones can serve as

Tidal wetlands are important blue carbon reservoirs, but it is unclear how sea-level rise (SLR) may affect carbon cycling and soil microbial communities either by increased inundation frequency or via shifting plant species dominance. We used an in-situ marsh organ experiment to test how SLR-scenarios (0, +7.5, +15 cm) and vegetation treatments (Spartina alterniflora, Spartina patens, Phragmites australis, unvegetated controls) altered CO2 fluxes (net ecosystem exchange, ecosystem respiration), soil carbon mineralization rates, potential denitrification rates, and microbial community composition. Increasing inundation frequency with SLR treatments decreased the carbon sink strength and promoted carbon emissions with +15-cm SLR. However, SLR treatments did not alter soil chemistry, microbial process rates, or bacterial community structure. In contrast, our vegetation treatments affected all carbon flux measurements; S. alterniflora and S. patens had greater CO2 uptake and ecosystem respiration compared to P. australis. Soils associated with Spartina spp. had higher carbon mineralization rates than P. australis or unvegetated controls. Soil bacterial assemblages differed among vegetation treatments but shifted more dramatically over the three-month experiment. As marshes flood more frequently with projected SLR, marsh vegetation composition is predicted to shift towards more flood-tolerant S. alterniflora, which may lead to increased CO2 uptake, though tidal marsh carbon sink strength will likely be offset by increased abundance of unvegetated tidal flats and open water. Our findings suggest that plant species play a central role in ecosystem carbon dynamics in vegetated tidal marshes undergoing rapid SLR.

250 words) 1 Development of sudden vegetation dieback (SVD), a phenomenon that causes 2 the rapid mortality of salt marsh plants, specifically Spartina alterniflora, has 3 affected large-scale alterations in Atlantic coastal systems, through the often-4 complete removal of vegetation. In this study, two wetlands that differ in the time 5 since development of SVD were compared in order to study biogeographic and 6 temporal patterns that structure coastal wetland microbial communities and their 7 response to disturbance. 8Biogeographic and edaphic factors that distinguished the two wetlands, such 9 as differing salinity, water content, and soil carbon and nitrogen between the sites 10 were more strongly associated with sediment microbial community structure than 11 either sampling date or SVD development. In fact, no OTUs differed in abundance 12 due to the season samples were collected, or vegetation loss due to SVD. This is not 13 to say that SVD did not alter the composition of the microbial communities. The 14 taxonomic composition of sediment communities in SVD-affected sediments was 15 more heterogeneous between samples and a small number of OTUs were enriched 16 in the vegetated sediments. Yet, these data suggest that coastal wetland sediment 17 communities are predominantly shaped by environmental conditions and are 18 generally resilient to temporal cycles or ecosystem disturbances. 19 20 Importance (150 words) 21One of the challenges of microbial ecology is predicting how microbial 22 communities will respond to ecosystem change. Yet, few studies have addressed whether 23 3 microbial responses to disturbance are consistent over space or time. In this study we 24 employ SVD as a natural vegetation removal experiment and compare the sediment 25 microbial communities between two geographically separated wetlands (ca 125 km). In 26 this manner, we uncover a hierarchical structuring of the microbial communities, being 27 predominantly governed by biogeography, with lesser effects due to disturbance, or 28 temporal dynamics. 29In the present study, we compared sediment microbial communities between two 71 salt marshes both experiencing current outbreaks of SVD. However, the time since SVD 72 development differed between the two sites (5 versus. 10 years). To examine the relative 73 role of vegetation, we examined sediment microbial communities in summer (July) 74 during peak plant activity, and in fall (October) when salt marsh plants begin senescence. 75 field sampling. This work was supported by the USDA National Institute of Food and 388 Agriculture, Hatch project 1006211. 389 390 391 392 References 393 1. Mcleod E, Chmura GL, Bouillon S, Salm R, Björk M, Duarte CM, Lovelock CE, 394 Schlesinger WH, Silliman BR. 2011. A blueprint for blue carbon: toward an 395 improved understanding of the role of vegetated coastal habitats in sequestering 396 CO 2. Front Ecol Environ 9:552-560. 397 7. Armitage A, Fourqurean J. 2016. Carbon storage in seagrass soils: long-term 409 nutrient history exceeds the effects of near-...