Organic matter accumulation by alkaline-constructed soils in heavily metal-polluted area of Subarctic zone

The research aimed to investigate properties and functions of soils constructed from alkaline mining wastes of different origin to remediate the industrial barren resulted from long-term emissions of the copper-nickel factory in the Subarctic region (Kola Peninsula, Russia). Conventional indicators of the remediation effectiveness (pH and metal content in geochemical fractions) were related to the indicators of soil functions such as biomass production, accumulation of organic carbon, microbial activity, and soil respiration. The experimental area included two sites with polluted and degraded Podzol and Histosol soils located in 1.5 and 0.7 km from the nonferrous (Cu-Ni) smelter, respectively. At the sites, artificial soil constructions were made from mining wastes or quarry sand covered by the vermiculite layer with lawn grasses planted on top. Plant biomass was collected every year starting from the experiment set-up. In 5 to 8 years, soil samples were collected on the layer basis, and chemical, biological, and morphological properties were analyzed. Sequential fractionation of metals was conducted using a modified Tessier’s scheme. The microbial biomass and its respiration activity were determined. Micromorphological studies were conducted using an optical microscope. Soil respiration was measured on-site by IRGA with simultaneous observations of soil moisture and temperature. The plant growth and residues' deposition at both experimental sites triggered carbon accumulation and resulted in 2–3 times higher content of organic carbon in the upper constructed soil layer compared to the initial content in mining wastes. Carbon accumulation was a key driver for the development of soil microbial communities and had a positive effect on the metal immobilization. This effect was strengthened by high pH inherited from the alkaline wastes and resulted in the performance of constructed soils as geochemical barriers. In their upper layers, where the root biomass was the highest, about 30–60% of Cu and Ni were bound by organic matter. In the underlying polluted soil, the most toxic water-soluble metal fraction was completely neutralized; and the metal concentrations in exchangeable fraction decreased by a factor of four improving the habitat conditions of the microbiome. Organic matter accumulation by clay material with the formation of organo-mineral films was found in the vermiculite-lizardite variant. Soil constructions made from alkaline mining wastes in the Subarctic supported the development of plant and microbial communities, organic matter accumulation, and metal immobilization. This technology allows protecting the environment from further pollution under the continuous emissions of the copper-nickel factory.