Evaluation of immobilizing agents as soil quality conditioners in addition to their metal(loid) immobilizing effect

In trace metal (TM)-contaminated agricultural soils, management of TM availability is important for safe crop production. In addition, maintenance or improvement of soil quality is vital for sustainable crop cultivation. Decreased TM phytoavailability and increased soil quality can be achieved by the application of various immobilizing agents to soil, which can supply both macronutrients and organic matter. This study investigated the long-term influences of four common immobilizing agents on soil biogeochemical properties and the phytoavailability of TMs in mixed metal-contaminated soil from a cultivated upland near an abandoned mining site. Lime (L), gypsum (G), fly ash (F), and animal manure-based compost (C) were applied to pots containing contaminated soil, either individually or in combination. After incubation for three years under sequential cultivation of two crops and fallow, soil biogeochemical properties were determined, and Brassica rapa plant bioassay was performed. The phytoavailability of all TMs (both cationic metals and anionic metalloids) remained significantly lower in soils treated with immobilizing agents even after three years, when compared with the no-agent control (CK) soil. In addition, the soil quality was significantly improved by treatment with immobilizing agents. For instance, the C and L + C treatments were the most effective in improving soil physical (bulk density, porosity, and water-resistant aggregate stability), chemical (pH, organic matter, total nitrogen, cation exchange capacity, and plant-available phosphorus, magnesium, and potassium), and biological (microbial biomass carbon and dehydrogenase activity) properties. The improvement of soil properties and lowering of TM bioavailability were also consistent with the most significant increase in B. rapa biomass production observed in the C treatment, followed by the L + C, G + F, L, G, F, and L + G treatments, as compared with that in CK. These results indicate that the function of the TM-immobilizing agent as a soil quality conditioner, in addition to its TM immobilizing effect, should be considered when selecting such agents for agricultural or ecological applications.