Unveiling the hidden consequences of mining tailings on ecosystem processes: Disruption of decomposition of native species in the Rio Doce watershed

The Fundão dam collapse deposited ∼50 million m³ of iron ore tailings into the Rio Doce watershed, severely altering soil properties and riparian vegetation. Eight years after the disaster, we assessed how soil containing mining waste affected decomposition dynamics, a key ecosystem process for nutrient cycling and forest recovery. This study was conducted in the Rio Doce watershed (Brazil), across five regions (Mariana, Rio Casca, Ipatinga, Conselheiro Pena, and Aimorés) and two seasons (dry and rainy), comparing reference and tailings-impacted soils. We employed two approaches, the standardized Tea Bag Index (TBI) to measure decomposition rate (k) and stabilization factor (S), and a modified TBI method to measure litter decomposition rates from three dominant native trees species per region, plus the invasive grass Urochloa decumbens. The standardized TBI (k and S) did not differ significantly between impacted and reference sites, indicating low sensitivity of this method to soil contamination. By contrast, decomposition of native litter was highly responsive. We found that Piptadenia gonoacantha exhibited a 55 % reduction in k at impacted sites, Schinus terebinthifolius showed 63 % lower k in impacted soils and 24 % lower in the dry season, and Eugenia florida decomposed 66 % faster in the dry season than in the rainy season. Soil properties such as Fe, P, potential acidity, and texture were significant drivers of these species-specific responses. Critically, the invasive grass U. decumbens maintained or accelerated decomposition in impacted soils, suggesting that altered edaphic conditions may favor its spread. Our results demonstrate that while TBI may underestimate contamination effects, species-specific litter decomposition captures critical ecological responses. For restoration, integrating native species with contrasting decomposition strategies can enhance nutrient cycling, while controlling invasive grasses is essential to avoid their advantage in contaminated soils. These findings provide actionable insights for riparian forest recovery in post-mining landscapes.