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Oxidized iron (Fe) can reduce seagrass dieback when present in sufficient quantities in the sediment to fix sulfide as pyrite (FeS₂) or iron monosulfide (FeS). However, the oxidized Fe pool may become depleted over time as Fe is reduced and precipitated with sulfides. In this study, we estimated long-term variations in the speciation of solid forms of reduced and oxidized Fe along a eutrophication gradient in West Falmouth Harbor (WFH) (a temperate lagoon with substantial seagrass meadows) and conducted a 6-week microcosm study to assess the role of oxidized Fe in supporting seagrass recovery. We planted seagrass in sediments obtained from 2 WFH regions with differing Fe speciation. We found depletion of oxidized Fe over a decade following a seagrass dieback, even when the soluble sulfide levels decreased to concentrations unlikely to cause toxicity in seagrass. The continued absence of large concentrations of available oxidized Fe minerals in sediments, where most Fe was bound in FeS₂, could impede the recovery of seagrass in formerly vegetated regions. Seagrass grown in sediments with low Fe:S ratios exhibited an increased probability of survival after 4 weeks. Field and laboratory results indicated that even when the soluble sulfide levels decrease after seagrass dieback, sediments may not be able to support seagrass recovery due to the legacy effects of eutrophication on the sediment Fe pool. However, we observed signs of reoxidation in the Fe pool within a few years of seagrass dieback, including a decrease in the total sediment S concentration, which could help spur recolonization. 

Abstract As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N₂fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N₂fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N₂fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of¹⁵N₂into biomass and a novel application of the N₂:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N₂fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m⁻² d⁻¹), with the highest rates (2.6 mmol N m⁻² d⁻¹) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N₂fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m⁻²d⁻¹), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N₂fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow. 

Abstract High rates of biological nitrogen fixation (BNF) are commonly reported for tropical forests, but most studies have been conducted in regions that receive substantial inputs of molybdenum (Mo) from atmospheric dust and sea‐salt aerosols. Even in these regions, the low availability of Mo can constrain free‐living BNF catalyzed by heterotrophic bacteria and archaea. We hypothesized that in regions where atmospheric inputs of Mo are low and soils are highly weathered, such as the southeastern Amazon, Mo would constrain BNF. We also hypothesized that the high soil acidity, characteristic of the Amazon Basin, would further constrain Mo availability and therefore soil BNF. We conducted two field experiments across the wet and dry seasons, adding Mo, phosphorus (P), and lime alone and in combination to the forest floor in the southeastern Amazon. We sampled soils and litter immediately, and then weeks and months after the applications, and measured Mo and P availability through resin extractions and BNF with the acetylene reduction assay. The experimental additions of Mo and P increased their availability and the lime increased soil pH. While the combination of Mo and P increased BNF at some time points, BNF rates did not increase strongly or consistently across the study as a whole, suggesting that Mo, P, and soil pH are not the dominant controls over BNF. In a separate short‐term laboratory experiment, BNF did not respond strongly to Mo and P even when labile carbon was added. We postulate that high nitrogen (N) availability in this area of the Amazon, as indicated by the stoichiometry of soils and vegetation and the high nitrate soil stocks, likely suppresses BNF at this site. These patterns may also extend across highly weathered soils with high N availability in other topographically stable regions of the tropics.