Reforestation with four native tree species after abandoned gold mining in the Peruvian Amazon

Abstract Global demand for gold has led to a massive increase in mining activity around the world. During the last decade, gold mining grew significantly in the Amazon becoming a major driver for land degradation and heavy metal contamination. However, few studies have explored soil degradation, reforestation, and plant mercury accumulation after mining operations. In this study, we established a reforestation field experiment in a gold mined area. We tested the outcome of planting seedlings of four native tree species previously grown in nursery polyethylene bags versus planting bare root seedlings, as well as the effect of three levels of biofertilization on seedling survival and growth. Previous to the experiment, we evaluated the level of soil degradation by comparing physical and chemical soil properties between the mined area and the nearest undisturbed reference forest. One year after planting, we also sampled roots, stems, and leaves of the planted species in order to detect possible mercury (Hg) accumulation in plant tissues. Our results revealed that soil texture becomes disproportionately sandy, while organic matter content and cation exchange capacity were seven- and three-fold lower in the mined area than in the reference forest, respectively. Seedling survivorship and growth varied across planting methods, biofertilization intensity, and species. Even in the bare root planting technique seedling survivorship was highly acceptable (75%) and increased with transplanting (83%) and the addition of biofertilizer (92%). Although seedling growth was improved significantly by the addition of diluted and pure biofertilizer, overall growth was found to be poor. Two individuals – distant from each other – out of a total of 60 sampled, showed traces of total Hg. A stem from Ceiba registered 8.52 mg Hg/kg and the roots of an Erythrina presented 0.60 mg Hg/kg. Total estimates of reforestation costs ranged between $1662 and $3464 per hectare in year 1. Poor soil fertility, slow species growth rates, and traces of Hg in plant tissues indicate that remediation and restoration in areas degraded by gold mining can be very challenging.