Effect of biochar on the extractability of heavy metals (Cd, Cu, Pb, and Zn) and enzyme activity in soil.

Biochar is a carbon-rich solid material derived from the pyrolysis of agricultural and forest residual biomass. Previous studies have shown that biochar is suitable as an adsorbent for soil contaminants such as heavy metals and consequently re- duces their bioavailability. However, the long-term effect of dif- ferent biochars on metal extractability or soil health has not been assessed. Therefore, a 1-year incubation experiment was carried out to investigate the effect of biochar produced from bamboo and rice straw (at temperatures ≥500 °C) on the heavy metal (cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn)) extract- ability and enzyme activity (urease, catalase, and acid phospha- tase) in a contaminated sandy loam paddy soil. Three rates (0, 1, and 5 %) and two mesh sizes (<0.25 and <1 mm) of biochar applications were investigated. After incubation, the physico- chemical properties, extractable heavy metals, available phos- phorus, and enzyme activity of soil samples were analyzed. The results demonstrated that rice straw biochar significantly (P<0.05) increased the pH, electrical conductivity, and cation exchange capacity of the soil, especially at the 5 % application rate. Both bamboo and rice straw biochar significantly (P<0.05) decreased the concentration of CaCl2-extractable heavy metals as biochar application rate increased. The heavy metal extractability was significantly (P<0.01) correlated with pH, water-soluble organic carbon, and available phosphorus in soil. The 5 % appli- cation rate of fine rice straw bioc har resulted in the greatest re- ductions of extractable Cu and Zn, 97.3 and 62.2 %, respectively. Both bamboo and rice straw biochar were more effective at de- creasingextractableCu andPb thanremoving extractable Cdand Zn from the soil. Urease activity increased by 143 and 107 % after the addition of 5 % coarse and fine rice straw biochars, respectively. Both bamboo and rice straw biochars significantly (P<0.05) increased catalase activity but had no significant im- pact on acid phosphatase activity. In conclusion, the rice straw biochar had greater potential as an amendment for reducing the bioavailability of heavy metals in soil than that of the bamboo biochar. The impact of biochar treatment on heavy metal extract- ability and enzyme activity varied with the biochar type, appli- cation rate, and particle size.