Root turnover and soil indicators capture belowground recovery following saltmarsh restoration

Abstract. Coastal wetlands, including saltmarsh, are highly productive ecosystems, with carbon- and nutrient-rich soils supporting biodiversity. Beyond carbon stocks and sequestration, the responses to restoration of these nutrient-rich and structurally complex soils remain poorly defined for coastal wetlands, especially in saltmarsh ecosystems restored by exclusion fencing. This study used a space-for-time approach to evaluate belowground responses in Salicornia quinqueflora-dominated saltmarsh 25 years after ungulate exclusion in Swan Bay, Victoria, Australia. We monitored surficial soil physicochemical characteristics, root and standardised litter decomposition, and root molecular composition across Grazed, Restored, and Natural Reference sites. Restored and Reference sites had ≥20 % higher vegetation cover and 2–3-fold higher percent soil carbon and nitrogen content, with 2.5-fold lower shear vane soil strength compared to Grazed sites. However, carbon and nitrogen stocks in the top 10 cm were not significantly different across sites (means ranging 30–36 Mg C ha^-1) due to elevated bulk density at Grazed sites caused by compaction from ungulates. Salicornia quinqueflora root litter decomposition was slowest in Natural Reference sites, with molecular composition showing preservation of recalcitrant lignin in the Reference and Restored sites, indicating greater soil carbon preservation capacity. In contrast, roots decomposing in Grazed sites showed increased nitrogen and phenolic compounds indicating greater microbial-driven turnover. This study demonstrates that exclusion fencing can restore saltmarsh soil function and promote long-term resilience, particularly through improved preservation of recalcitrant organic matter material decades after intervention. By highlighting shifts in surface soil structure and organic matter preservation, this study shows why soil quality metrics beyond carbon stocks are essential for accurately evaluating restoration outcomes.