Changes of soil heavy metal contents and pollution evaluation during the restoration of wetlands

ObjectiveThis study was designed to evaluate the effects of converting farmland to wetland on the contents of heavy metals in soil.

MethodWe measured heavy metal contents of the soil in soybean fields, restored reclaimed wetlands and natural Carex schmidti-Deyeuxia angustifolia marsh in Heilongjiang Sanjiang National Nature Reserve of northeastern China. We also evaluated the effects of converting farmland to wetland on the potential ecological hazard by the Hankanson potential ecological hazard index method.

ResultThe results showed that the highest heavy metal pollutant in the soil of restored sites in this reserve was Pb, followed by Cu, while the effects of Zn, Mn and Cr were small. The contents of Cu, Pb, Zn, Mn and Cr fluctuated with the soil deepened. The contents of Cu and Pb were highest in the soil layer of 0−10 cm during the begining 11-years restoration. However, those were the highest in the soil layer of 40−50 cm after 11-years restoration. Other heavy metal contents had no certain trends along with soil depth. The contents of Cu, Pb, Zn and Cr in soil increased first and then decreased with prolonged restoration time, while the increasing and reducing of Zn content were not reaching significant level (P > 0.05). The contents of Mn in restoration sites were significantly lower than that in soybean field (P < 0.05) from the first year of restoration. The soybean fields and plots within 6-years restoration had strong potential ecological risk hazards. The potential ecological risk hazard indexes of sample plots after 8-years, 11-years restoration as well as C. schmidti- D. angustifolia marsh were moderate, while that of sample plots after 15 years restoration was mild.

ConclusionThe contents of Cu, Pb, Zn and Cr in soil increase first and then decrease with prolonged restoration time. Those contents reach the level of natural wetlands after 15, 12, 2 and 10 years restoration, respectively. However, there is no significant differences in soil Mn content between restored wetlands and natural marsh (P > 0.05). The potential ecological risk of ecosystems reduces gradually with the restoration time extends.