Restoration type determines synchronic recovery of soil carbon, nitrogen, and phosphorus in mangrove wetlands

Restoration is essential for preserving the functions and economic benefits of mangrove ecosystems. Soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) can help assess restoration effectiveness. However, it remains unclear whether active restoration (AR) with planting better recovers these nutrients than passive restoration (PR) without planting. We measured SOC, TN, and TP in four soil layers (0-10, 10-20, 20-30, and 30-40 cm) in experimental plots restored using AR and PR pond-to-mangrove methods over three years. The trials were compared to nearby natural mangrove forests in the Qinglan Harbor area of Hainan Island, China. We found that the SOC, TN, and TP contents in restored mangroves were significantly lower than those in natural mangroves, highlighting the long-term nature of ecosystem recovery. However, no significant differences were observed in SOC (10.40 ± 0.71 vs. 8.95 ± 0.54 g kg^-1), TN (0.47 ± 0.04 vs. 0.44 ± 0.03 g kg^-1), and TP (0.14 ± 0.01 vs. 0.09 ± 0.01 g kg^-1) contents between AR and PR sites. This challenges the common assumption that AR is always superior to PR. 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 levels. Our findings suggest that both AR and PR are viable restoration options, depending on ecological needs, economic resources, and sustainability goals.