Long-term organic fertilization shields soil prokaryotes from metal stress while mineral fertilization exacerbates it

Metal contamination in agricultural soils threatens prokaryote dynamics essential for soil health and crop productivity. Yet, whether fertilization in the long-run affects their resilience to metals remains unclear. This study examined the biogeochemical impacts of realistically low-dose applications of cadmium, zinc, and lead in soils subjected to 119 years of non-fertilization, mineral-fertilization (NPK), organic-fertilization (manure), or combined mineral-organic fertilization. Amended metals remained in the mobile fraction with the order: mineral < unfertilized < mineral + organic < organic, mirroring the effects on soil prokaryotes. In both unfertilized and mineral-fertilized soils, 16S rRNA gene copy numbers declined by 30 % upon metal addition, but recovery timing differed: in unfertilized soil, recovery began after three days, whereas in mineral-fertilized soil, numbers declined until day seven before recovering. This coincided with an increase in metal-resistant taxa, particularly in mineral-fertilized soil, with 10 significantly affected OTUs, and to a lesser extent in unfertilized soil, with 5 affected OTUs. Carbon-, nitrogen-, and phosphorus-mining enzyme activities increased 50-100 % in mineral-fertilized soils, suggesting enhanced nutrient acquisition to mitigate metal toxicity. In contrast, organic-fertilized soil hosted stable enzymatic activities and microbial copy numbers with minimal community shifts (1 affected OTU), indicating greater resistance to metal amendment. Combined mineral-organic fertilization stabilized copy numbers and enzymatic activity upon metal amendment, but 8 OTUs were affected, including specialized nutrient cyclers, suggesting increased availability of previously adsorbed NPK cations. Our findings indicate that organic fertilization shields prokaryotes from metal stress, while mineral fertilization exacerbates it, highlighting the benefits of organic practices for maintaining soil health and productivity.