INTRODUCTION
Gold industry in the world has experienced a surge over recent years with the record mine production of 4,490t (World Gold Council, 2018a) and reaching great international price of 1,456.85$ OZ TR-1 (World Gold Council, 2019). Peru occupies 1st place in Latinoamerica and the 6th place in the world gold production, which represents 4.4% overall world gold production (Ministerio de Energía y Minas, 2019). Gold is an extremely scarce element (World Gold Council, 2018b), which in the Amazon need the extraction of thousands tons of sediments to get goldbearing gravel, leading to a large scale deforestation and Hg pollution (Alvarez, Solano, Brack, & Ipenza, 2011). In Madre de Dios Region, also known as the “Peruvian Capital of Biodiversity”, artisanal and small gold production accounts for 7% of the total annual gold production in Peru of approximately 144 metric tonnes (Ministerio de Minas y Energía, 2019). Deforestation caused by gold mining industry has become a major threat to some of the most remote and better-conserved old-growth forests in tropical South America. Between 2001 and 2013, there was a loss of 1680,000 ha of forest at 177 gold mining sites (Alvarez-Berríos & Mitchell Aide, 2015). Here the Peruvian Amazon constitutes part of the highest biodiversity region in the global tropics (Myers, Mittermeier, Mittermeier, da Fonseca, & Kent, 2000). Unfortunately, gold mining activities in Madre de Dios has resulted in the deforestation of 95,750 ha (Caballero Espejo et al., 2018), annual deforestation rates have fluctuated between 6,000 ha (Asner, Llactayo, Tupayachi, & Luna, 2013) leading to an estimated topsoil loss of 1.3 t ha-1year (Gomez, 2013). Moreover, It was reported that gold market prices were close correlated with mining rates on interannual timescales (R2 =0.43; p < 0.05) (Asner & Tupayachi, 2017). This pattern of mining in tropical forested regions continues to rise globally with similar consequences as observed in Madre de Dios (Alvarez-Berríos & Mitchell Aide, 2015). Alluvial gold mining in Madre de Dios old growth forest generally involves slash and burn deforestation, sediment extraction, amalgamation of gold with Hg, burning, Hg evaporation and gold recovery (Alvarez et al., 2011; Salinas, 2007). All these stages of gold production are typically carried out on site, hence generating an importance source of Hg pollution in the local environment. It is estimated that between at least 1 and 2 grams of Hg is lost to the environment for every gram of produced gold (Mallas & Benedicto, 1986; Veiga & Baker, 2004). Sediment extraction is accomplished through the use of highly mechanized mining and minimally mechanized technology level. Highly mechanized mining uses heavy machinery such as excavators, front loaders, and dump trucks, and minimally mechanized mining uses suction pumps, high pressure water cannons to liquify stream-side sediments which are transported to sluice boxes via diesel-powered water/sediment pumps (Caballero Espejo et al., 2018). During this process the topsoil, characterized by a fine texture, is scattered and coarse gravel, stones and boulders from deeper soil layers become to predominate at the surface (Salinas, 2007). In spite of the degraded conditions of mine spoils, natural succession also happens in most of them resulting in the formation of secondary forests in different stages of development, and with it the formation of new soil profiles with different physical and chemical characteristics. However, limited information exists in the Amazon about the composition of such new soil profiles, hampering decision making on how to best reclaim, rehabilitate or restore the ecosystem in these areas. Our aim was to support further effective restoration experiences by classifying these soils in Madre de Dios according to the Soil Taxonomy (Soil Survey Staff, 2014) and World Reference base (Working Group WRB, 2015) to verify how they differ from soils under undisturbed old-growth forest. We primarily focused on a local area, such as Fortuna Community, which is the most characteristic model of Artisanal and Small Scale Gold Mining (ASGM) in the Peruvian Amazon.