Study of soil physicochemical properties and heavy metals of a mangrove restoration wetland

To better understand the optimal restoration pattern, this study explored the soil physicochemical properties and distribution characteristics of heavy metals under various planting patterns in the mangrove restoration wetland which in Jinjiang, Fujian Province. Based on 30 soil samples collected from the wetland in April 2019, we thoroughly analyzed the distribution characteristics of soil physicochemical properties (particle size (PS), pH, soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP)) and nine heavy metals elements (Cr, Ni, Cu, Zn, Pb, Cd, As, V, Co) under different planting patterns. Pollution levels were also studied, and heavy metal concentrations in soil were investigated by correlation analysis, with relevant experimental data from April 2016 and April 2017 used in the comparative analysis. The results showed that in the entire study area, the concentrations of SOM, TN and TP in the low-density area were 35.8%, 18.03%, and 21.36% higher than those in the high-density area, respectively. In the plot of “A - K–C, Acanthus ilifolius - Kandelia obovate - Aegiceras corniculatum”, the concentrations of SOM, TN and TP were more easily accumulated. The trend in average concentrations of 9 heavy metals were as follows: Zn > V > Pb > Cr > Cu > Ni > As > Co > Cd. The concentration of heavy metals in the soil of the “A - K–C, Acanthus ilifolius - Kandelia obovate - Aegiceras corniculatum, 0.5 × 0.5 m (II#3)” plot was the lowest, and a preliminary conclusion that the II#3 pattern for the restoration of soil heavy metal has a more significant role. Based on the Geo-accumulation Index (Igeo) and the Potential Ecological Risk Index (Eri), the pollution levels of the 8 elements other than Cd either did not reach high levels or had no pollution. Moreover, pH value was negatively correlated with SOM at 5% confidence level, and the correlation coefficient was −0.354 (p < 0.05). The TP increased but the TN decreased year by year, and we can thus consider that the wetland soil and the activated sludge have the same nitrification and denitrification, which is worth for further study. With the increase of wetland restoration time, heavy metals were gradually removed; the concentrations of Cu, Cr, Ni, Pb, Cd in the fifth year were significantly lower than before, and the pollution levels were also significantly reduced. The research methods and results can provide theoretical and scientific bases for further study of wetland vegetation and can help in choosing feasible restoration methods for further wetland restoration.