Organic amendments influence soil properties, soil microbial diversity, and winter barley traits in a long-term field trial with contaminated soils at a former wood preservation site

Soil contamination with metal(loid)s and organic pollutants poses significant risks to the environment and human health, driving the need for sustainable remediation strategies. Organic amendments can mitigate contamination effects, enhance soil quality, and potentially increase biomass production but their long-term influence, depending on their quality, remain an open question. This study investigates the long-term effects (year 5) of five organic amendments - fresh pig manure (PM), biodigested pig manure (PD), compost (C), compost pellets (P), and green waste compost (G) - on Cu-contaminated soils at a former wood preservation site. Here, we evaluated their impacts on physico-chemical soil properties, metal bioavailability, microbial community structure, plant growth and soil fertility. All amendments resulted in overall soil improvement, including enhanced physico-chemical soil properties, as well as increased enzyme activities related to carbon, phosphorus, and sulphur mineralization. The amendments promoted the concentration of soil 16S bacterial genes and improved the yield of winter barley cultivated on the plots. The most abundant phyla detected across soil samples were Actinobacteriota, Proteobacteria, and Firmicutes, with Bacillus, Streptomyces, and Bradyrhizobium among the dominant genera. Compost-based amendments at 5% w/w addition rate (C5 and P5) showed the most promising results, significantly increasing soil carbon, nitrogen, and phosphorus levels, while reducing bioavailable metals (Cd, Ni, Pb, and Zn) compared to untreated control plots (Unt, p < 0.01); a drop in Cu availability was also noted but its significance was attenuated by its spatial variability in the Unt plots. The P5 soils exhibited the highest 16S rRNA gene copy number (p < 0.01). Both compost (C) and compost pellets (P) amendments enriched microbial communities associated with soil quality and plant yield, leading to significant improvements in soil fertility and barley yield (+200% on average). This integrative approach identified organic amendments, notably compost and pelleted compost, that effectively contribute to soil remediation from multiple perspectives - chemical properties (pH, organic content, and nutrients), metal bioavailability (Cd, Cu, Ni, Pb, and Zn), enzyme activities, microbial abundance and diversity (16S rRNA), and winter barley yield. The study evidenced signature biomarkers characteristic of healthy soils (Paenibacillus, Lysinibacillus, Agromyces, Limnochordaceae, and Ureibacillus) and polluted soils (Candidatus Solibacter, Mycobacterium, and Conexibacter). Our findings support the use of organic amendments as a balanced approach for soil remediation, promoting reduced metal exposure while enhancing microbial health and soil fertility.