Synchronic recovery of soil carbon, nitrogen, and phosphorus depends on restoration type in mangrove wetlands

Restoration has emerged as a critical need to ensure the critical function and economic benefits of mangrove ecosystems. Soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) can help assess the effectiveness of restoration. However, it remains unclear whether active restoration (AR) with planting results in better recovery of these nutrients than does passive restoration (PR) without planting (natural regeneration). We empirically assessed the SOC, TN, and TP contents in four soil layers (0‒10, 10‒20, 20‒30, and 30‒40 cm) in the Qinglan Harbor mangroves of Hainan Island, China. We compared sites subjected to different pond-to-mangrove restoration methods over three years (AR and PR) to nearby reference sites with natural mangrove forests. We found that the SOC, TN, and TP contents in restored mangroves were considerably lower than those in natural mangroves, highlighting the long-term nature of ecosystem recovery. However, the nutrient content did not differ between the AR and PR sites. The scaling slopes of the C:N:P stoichiometric relationships remained consistent (slope =1) across the whole study area and at each site and soil depth, indicating tight coupling of these elements post-restoration. Soil salinity and bacterial community richness were identified as significant determinants of nutrient level. Consistent stoichiometric slopes across conditions imply a synchronized recovery of soil nutrients that is crucial for the sustainable management and restoration of mangrove ecosystems.