Physical protection by soil aggregates drives carbon gains in mangrove restoration: Evidence from coastal aquaculture ponds.

Mangrove wetlands are vital carbon sinks, yet their area has sharply declined due to aquaculture. To evaluate soil carbon stock recovery, we analyzed six pond-to-mangrove restoration sites along China's southeastern coast, assessing changes in carbon components and their drivers. Restoration increased soil water content, salinity, and total nitrogen while reducing pH and bulk density. Soil organic carbon (SOC) and its labile fractions significantly increased, with both manual and natural restoration improving carbon dynamics. SOC positively correlated with water content, total nitrogen, and total phosphorus but negatively with pH (p < 0.01). Crucially, we found that the physical protection mechanism of soil aggregates was the dominant driver of soil carbon stability (38.5 %), followed by environmental factors (33.6 %), microbial bioprotection (19.6 %), and chemical protection (8.30 %). We roughly estimated that converting coastal aquaculture ponds to manually restored mangrove wetlands increased SOC storage in the 0-30 cm layer by approximately 160 t C km, while natural restoration led to a larger increase of about 212 t C km. We suggest that mangrove wetland restoration prioritize natural processes, with artificial methods as a supplementary approach. These findings support effective mangrove conservation and restoration, enhancing climate resilience and ecological-economic benefits.