## 1 Introduction  
Open-pit coal mining is one of the human activities with the greatest environmental and social impact since it generates changes in land use in large areas, affecting ecosystems and biodiversity [1]. Ecological rehabilitation of these areas involves the restoration of ecosystem processes that sustain the provision of goods and services, particularly those generated by the soil [2, 3]. A basic component that must be evaluated in soil under restoration processes is the soil macrofauna [4], which plays a fundamental role in maintaining ecosystem functions [5].  
Ants, termites, and earthworms are considered ecosystem engineers because they affect soil structure by the formation of soil aggregates and pores, and by the production of physical structures that can modify the availability of and access to resources for other organisms [6]. Ants are particularly important regulators of soil processes on different temporal and spatial scales [7]. For example, the bioturbation generated during the construction of nests can change soil physical properties, affecting the soil porosity and water flow [8; 9]. Additionally, nest construction modifies the chemical composition of the soil by incorporating nutrients and organic matter from parts of insects, feces, and seeds, which act at different temporal and spatial scales [10; 11; 12]. These processes contribute to soil quality improvement in rehabilitated areas; however, in the mine context, ants have been mainly recognized as bioindicators [13; 14; 15; 16]. Nevertheless, it is necessary to know the importance of ants as ecosystem engineers and habitat disturbance, especially in the technosols of rehabilitated mines and forests undergoing restoration [17; 18].  
_Pheidole fallax_ is a predatory and carrion ant with Neotropical distribution, i.e., in the Greater Antilles, Central America, Colombia, Venezuela, and Brazil [19; 20]. Studies have observed that _P. fallax_ populations have many individuals cooperating during foraging, while the species has a worker that retrieves small particles of food and is responsible for mass recruitment [21]. They build nests (Figure 1) with a single entrance, with a gallery leading to several chambers near the surface of the ground, and a refuse pile on the outside [22]. These ants are some of the most abundant in sites under rehabilitation for long periods of time and in forests without mining [15]. The species also plays an important role as a seed disperser and predator [22]. Ants of the genus Pheidole (Subfamily: Mirmicinae) are considered effective ecosystem engineers; their nests can improve soil chemical characteristics through the transfer of substantial quantities of organic matter, like arthropod carcasses and plant debris [23].  
Considering the above information, in this study we aimed to test whether nests of _P. fallax_ influence soil chemical properties, to characterize the organic debris found in the refuse piles, and to describe nest architecture as proxy of the bioturbation effect in rehabilitated sites of the Cerrejon coal mine (La Guajira, Colombia). We investigated whether the accumulation of arthropod carcasses and seeds in refuse piles contributes to soil nutrient enrichment. Additionally, we aimed to contribute knowledge on _P. fallax_ by studying the structure of its nests. In summary, we aimed to (i) test whether _P. fallax_ nests have an effect on soil chemical properties, (ii) characterize the debris of seeds, arthropods, and other organic items found in the refuse piles, and (iii) describe nest architecture as proxy of the bioturbation effect in the soil.