Biochar amendments in a California salt marsh restoration reduced denitrification and supported distinct microbial community functions.

Accelerated sea level rise, combined with human-induced changes to hydrology and sediment transport, threatens the long-term persistence of coastal wetlands. Sediment addition is increasingly used to enhance wetland resilience to sea level rise, and incorporating biochar, a carbon-rich material produced through pyrolysis of organic matter, may enhance carbon storage, soil function, and vegetation recovery. We examined how biochar derived from Eucalyptus spp. feedstock influenced sediment properties, microbial communities, functional potential, and nitrate reduction pathways, with particular emphasis on differences between fresh and aged biochar and effects on nitrogen processing - a recognized data gap in biochar research. Fresh biochar had no influence on denitrification rates, but aged biochar was associated with substantially lower denitrification rates (by 88%); no detectable DNRA was observed. Biochar-amended sediments exhibited subtle differences in sediment properties and microbial communities, but had higher relative abundances of Cyanobacteria and Truepera (phylum Deinococcota) as well as greater sediment carbon content than restoration sediments. Inferred decomposition-related pathways differed significantly between biochar-amended and restoration sediments (F = 3.22, p = 0.026), with lower inferred sulfate reduction and greater inferred fermentation in biochar treatments. However, biochar amendments to low-organic-matter restoration sediments were also associated with reduced denitrification, indicating potential tradeoffs between carbon storage and nitrogen removal. Additional research is needed to determine the mechanisms underlying these responses and their implications for wetland restoration.