Functional equivalence in early-stage decomposition between restored and remnant salt marshes despite differences in associated microbial communities

Restoration efforts have been increasing to address global salt marsh loss, yet they rarely monitor the return of important ecosystem processes such as decomposition or the microbial communities that drive them. This study used a standardised litterbag experiment to compare decomposition rates and microbial diversity between restored (>10 years) and remnant salt marshes in the Georges River estuary, Australia. Green and rooibos tea bags served as litter proxies and were buried in replicate plots across three remnant and three restored salt marshes. Sediment properties including organic matter, chlorophyll, silt content, moisture content and temperature were analysed, and decomposition rate was quantified as mass loss of tea litter after 3- and 6- months. Microbial communities associated with green tea litter were characterised using amplicon sequencing of 16S rRNA and ITS regions. Remnant salt marshes had higher sediment organic matter and chlorophyll content, suggesting greater productivity and carbon-storage capacity. Decomposition rates varied by tea type and time but did not differ between remnant and restored salt marshes: green tea decomposed faster overall due to its labile carbon content. While microbial community structure differed between restored and remnant sites, their putative functions were similar, possibly due to abiotic leaching influencing early-stage decomposition. These findings indicate that restored salt marshes can sustain organic matter decomposition processes comparable to remnant salt marshes, although sediment carbon accumulation may occur more slowly. • Decomposition rates were similar in restored (>10 years) and remnant salt marshes, despite higher organic matter content in the latter. • Tea-associated fungal richness and diversity was higher in remnant salt marshes. • Tea-associated bacterial richness and diversity were similar between the salt marshes, but highest at the 6 months. • Microbial community structure differed between restored and remnant sites. • Restored salt marshes support organic matter decomposition rates at levels comparable to remnant marshes.