The Orchestration and Harmonics of Biogeochemical Cycles in a Southeastern USA Saltmarsh

Abstract Saltmarshes provide vital ecosystem services, including coastal protection and habitat for fisheries. While feedbacks influencing vertical sediment accretion in marshes are well‐documented, including those between relative sea level and primary production, relationships among nutrient cycles, flooding, and primary production remain less explored. This study presents a unique 30‐year data set from the North Inlet estuary, South Carolina, tracking monthly growth of the dominant macrophyte, Spartina alterniflora , and porewater concentrations of sulfide, ammonium, and phosphate. Our findings reveal correlated seasonal and decadal patterns of sulfides, nutrients, and plant growth, with periodicities linked to seasonal climate and decadal tidal cycles. S . alterniflora thrives in anaerobic sediments where sulfides are toxic byproducts of sulfate‐reducing bacteria (SRB) produced through the oxidation of organic acids. Dissolved organic compounds fluctuated seasonally in tandem with plant growth. Organic acids utilized by SRB are released as root exudates and produced via cellulosic biomass fermentation. SRB fix nitrogen and produce sulfides that solubilize phosphate, compensating for nutrients lost in drainage and rafting of aboveground biomass. By directly and indirectly producing the substrates, Spartina effectively regulates SRB activity, thereby orchestrating biogeochemical cycles and accumulating sulfides that inhibit competing species. Our results challenge the traditional view of sulfides as mere phytotoxins and saltmarshes as passive nutrient transformers. Instead, we propose a model wherein S . alterniflora sustains its growth and supports estuarine productivity by regulating its nitrogen and phosphorus supplies through a mutualistic relationship with SRB.