Effect of different grain sizes of hydroxyapatite on soil heavy metal bioavailability and microbial community composition

Heavy metal contamination of soils has become a major global environmental problem. Previous studies have reported that hydroxyapatite has been extensively used to immobilize soil heavy metals, and the grain sizes of amendments affect the bioavailability and geochemical stability of heavy metals. However, it is not clear how soil microbial diversity and community composition in these soils vary with the application of hydroxyapatite. A field study was conducted to determine the effect of different grain sizes of hydroxyapatite on Cu and Cd bioavailability and microbial community composition. Results showed that micro-hydroxyapatite (MAP), nanohydroxyapatite (NAP) and normal hydroxyapatite (AP) increased soil pH and decreased soil exchangeable acid and exchangeable Al. Compared with the control, MAP was the most efficient in decreasing the CaCl2-extractable Cu and Cd by 97.5 and 86.5%, respectively, and exchangeable Cu and Cd by 82.4% and 27.4%, respectively. The soil microbial community composition varied with the amendments of different sizes, and the soil bacterial diversity increased after the amendment applications. Analysis of the network structure showed that fewer operational taxonomic units were enriched or depleted between the MAP- and NAP-amended soils, which explained the similarity between the MAP- and NAP-amended soils. These results suggest that MAP may be the best material for amending heavy metal-contaminated soils when considering immobilization efficiency and soil biological characteristics.