Soil Microbial Community Abundance and Metal Immobilization in Contaminated Soils with Slow Pyrolyzed Biochars

Slow pyrolyzed biochar is considered as a vital amendment in soil metal immobilization. However, its feasibility in multi-metal contaminated soils has not been studied well. This study evaluated the immobilization of Pb and As in two contaminated soils by slow pyrolyzed biochars at 500℃ derived from umbrella tree (Maesopsis eminii) wood bark (WB, pH 9.6), cocopeat (CP, pH 10.3) and palm kernel shell (PKS, pH 6.9). Also, biochar effects on microbial community and dehydrogenase activity (DHA) were evaluated. Biochar phosphorous contents were 485, 302, and 274 mg/kg respectively in WB, CP and PKS. The silicon content in WB, CP and PKS was 7604, 11590 and 10310 mg/kg respectively. Metal contaminated soils were collected from croplands nearby two mining sites (site-1 and site-2) in Korea. Soils were incubated with 5% (w/w) biochar at 25℃ and 70% water holding capacity for 45 days. After the incubation period metal existing geochemical fractions were assessed by sequential extractions. Fatty acid methyl ester analysis (FAME) was employed to assess the changes in microbial community. Exchangeable Pb was found in both site-1 and site-2 soils, but exchangeable As was detected only in site-2 soil. Addition of WB showed the highest reduction of Pb (site-1, 77.7%; site-2, 91.5%) in exchangeable fraction followed by that of CP (site-1, 67.1; site-2, 81.1%) and PKS (site-1, 9.1%; site-2, 20.0%); however, except PKS the remain two biochars increased the exchangeable As in site-2 soil (WB, 84.6%; CP, 14.8%). Alkalinity and high phosphorous content of biochars might be the main reasons in Pb immobilization or As mobilization. The silicon content in biochars is likely to be another influencing factor in increasing As mobility in the soils. The DHA was significantly increased by WB and CP in site-2 soil, but biochars were not increased the DHA in site-1 soil. The total FAME content in site-1 soil also was not increased by the biochars, while only PKS significantly increased the total FAME in site-2 soil. The low mobile matter content in biochar due to the relatively high pyrolysis temperature might not provide readily available C sources for microbial proliferation in soils. Biochar derived feedstock could be a critical parameter of biochar feasibility in soil health improvement in multi-metal contaminated soils. This study was supported by National Research Foundation of Korea (NRF; NRF- 2015R1A2A2A11001432).