Effects of Wetland Plant Biochars on Heavy Metal Immobilization and Enzyme Activity in Soils From the Yellow River Estuary

Abstract

Estuarine wetlands provide a variety of ecosystem services, including carbon sinks, nitrogen removal, marine habitats, and climate regulation. However, many estuarine marshes are suffering from soil heavy metal pollution, which significantly affects soil enzyme activities that influence the carbon and nitrogen biogeochemical cycles in wetlands. To date, studies on the effects of wetland plant biochars on heavy metal adsorption and enzyme activity in estuarine wetland soil are limited. The purpose of this study was to assess the effects of wetland plant biochars on the enzyme activity in heavy metal contaminated soil. The biochars were produced from Phragmites australis (PB), Suaeda salsa (SB), and Tamarix chinensis (TB) under different pyrolysis temperatures and times. The detected pyrolysis products showed that the ash, pH, electrical conductivity, and carbon content of the biochars increased significantly, while the production rate of the biochars decreased with increasing pyrolysis temperature and time. The results of the adsorption experiments indicated that biochar addition could effectively reduce the concentration of Pb and/or Cd in the Pb2+/Cd2+ single or mixed solutions, but the Pb2+ and Cd2+ in the mixed solution indicated a competitive adsorption. A 30-day incubation experiment was conducted using salt marsh soil amended with different biochar application rates to investigate the short-term effects of biochar addition on Pb and Cd immobilization. The PB and SB significantly immobilized Pb within the first 15 days, but Pb remobilized within the next 15-day period. In contrast, TB addition did not significantly fix Pb. Moreover, biochar addition promoted the conversion of Cd from the residue to the less immobile fractions. The addition of three types of plant biochar had no significant effect on the urease activity in wetland soil but significantly increased soil sucrase activity. PB and SB significantly promoted catalase activity, while TB significantly inhibited soil catalase activity.