# Effects of Manure Waste Biochars in Mining Soils
## 1 Introduction  
Traditionally, mineral extraction and processing for metal production are associated with landscape modifications and an important environmental risk. Metals, metalloids, organic compounds, and salts may be transferred to the soil, water, and air, affecting the trophic chain [1]. Furthermore, impacts on soil derived from mining activities, including erosion, significant pH variations, runoff, or higher metal mobility, produce soil degradation and groundwater pollution risk. In addition, the characteristics of mining soils hinder the success of establishing a vegetative cover once the mines are closed. In order to mitigate the above-mentioned environmental impacts, new remediation and restoration techniques are required. Traditionally, soil excavation [2], leaching of pollutants [3], electro-migration [4], soil-flushing [5], or the use of barrier technology [6] are used for this purpose. Advances in the last years modified these techniques to improve them. However, most of them are still expensive and lead to total loss of soil fertility [7]. Alternative techniques, which are environmentally friendly and with low implementation costs, are gradually gaining public acceptance. The most classical and famous techniques arephytoremediation, biotransformation, and more recently, the use of amendments [7; 8; 9], including compost, sewage sludge, crop residues, urban wastes, or manures [10; 11; 12]. Organic wastes can be used as amendments in mining soils due to their high content in nutrients and organic matter, mainly lignin and cellulose, with functional groups, including hydroxyls or carboxyls that allow complexation with metals and reduce their mobility [13]. However, the high mineralization and decomposition rates of organic wastes in soils result in low amendment stability, resulting in the need for re-applying the amendment. In addition, the potential presence of toxic compounds in wastes such as metal(loids), polychlorinated biphenyls (PCBs), or polychlorinated hydrocarbons (PHAs) are associated with risks to soil biota [14; 15]. More recently, there is an increasing interest in biochar as a soil amendment [16; 17]. Biochar is a carbon-rich material obtained by pyrolysis or thermal treatment of biomass under restricted O\({}_{2}\) atmosphere. During biochar production, the thermal treatment of organic wastes allows the elimination of some toxic elements present in the feedstock [18]. In addition, biochar acts as a soil fertilizer, increasing nutrient supply and plant growth [19], and as an adsorbent, reducing the bioavailability of metals [20]. The effect of biochar in soils depends greatly on the biochar characteristics, which are influenced by pyrolysis conditions and feedstock characteristics [21; 22]. For this reason, an exhaustive biochar characterization is essential to understand the biochar effects in a particular soil. Biochars produced from plants show low nutrient content, which implies the use of additional fertilizers [22]. However, manure-derived biochar can be used as an organic amendment due to its high nutrient content. For example, [PERSON] et al. [23] prepared biochar (pyrolysis temperatures: 300-500 \({}^{\circ}\)C) from cattle manure, poultry litter, and pig manure with 1.2-4.7%, 1.2-2.3 g\(\cdot\)kg\({}^{-1}\), and 8-35% P, N, and organic carbon, respectively. [PERSON] et al. [24] produced biochars from pig manure, which were suitable for both agronomic and remediation purposes. In addition, previous works indicated that manure-derived biochars show higher metal adsorption capacity than plant-derived biochars [25]. [PERSON] et al. [26] observed a greater zinc immobilization using biochar from cattle manure than utilizing grain husk biochar, while [PERSON] et al. [27] found cadmium and lead immobilization higher than 51% and 68.8%, respectively, with respect to control soil in two soils amended with poultry manure biochar at rates of 5-10%. However, the reduction of the bioavailability and toxicity of metals in mining soils is not the only objective of soil remediation.