## 4 Discussion  
Aspects of the biology of _P. fallax_ have been described mainly in forest and agroecosystems. This work, along with that of other researchers [15; 22; 25], seeks to understand the role of this species in rehabilitated coal mine areas. The results of this study show that the bioturbation made by these ants can improve the soil conditions through nest building and maintenance, but also by accumulating organic debris. Data obtained from external refuse piles suggest that _P. fallax_ colonies use food resources of different trophic levels, with arthropods and seeds being the main foods in their diet. These results are consistent with results of other studies in which _P. fallax_ workers have been identified collecting seeds and insect carcasses, which are then carried inside the nests and later deposited in the refuse pile [23]. The greater amount of arthropod and seed remains found in refuse piles in the 20-year-old site, compared to the 16-year-old site, could indicate a greater supply of resources. It is important to highlight that the four additional years at the 20-year-old site are not the reason for the greater supply of resources. Maybe this difference is more related to better habitat conditions at the 20-year-old site, which promote the establishment of a greater number of nests both for this species [15; 25] and for others [14; 34]. Furthermore, species of the _Pheidole_ genus have small foraging areas (\(\pm 7\) m) [35], which would indicate that the resources collected are close to where the nests are located. The hauling of corpses and seeds to the nests could trigger the incorporation of nutrients into the soil, causing an enrichment of the adjacent edaphic horizons [17; 23]. This assertion coincides with the results of chemical analysis, where the composition of refuse piles was different when compared to the surrounding soil and interior of nests. In other words, high levels of organic matter showed that the entrance of the nest is an important location for the accumulation, translocation, and transformation of soil organic matter [23]. Refuse piles were also identified as sites with lower pH as compared to the surrounding soil. Other nutrients, such as P, K, N, and Mg, are also elevated in the refuse piles according to other studies with ant nests [36; 37; 38]. NIRS is a practical technique that allowed us to distinguish among refuse pile soil, soil from the interior of the nest, and soil surrounding the _P. fallax_ nests, thus evidencing modifications in soil organic matter content as a result of ant activity. Similar values for the nest interior and control soil may be explained by bioturbation and mixing of soil particles, leading to a reduction or homogenization of  
Figure 4: _P. fallax_ nests molded with dental gypsum, showing the ant nest architecture in rehabilitated sites of the Cerrejón mine in La Guajira, Colombia. the nutrient content [39; 40]. However, NIRS is a practical technique that allowed us to distinguish not only refuse pile soil from other types, but also soil from the interior of the nest from soil surrounding the _P. fallax_ nests, although the difference between these was not lower. This is evidence of modifications in soil organic matter content as a result of ant activity, supporting the usefulness of NIRS spectra in identifying the origin of biogenic structures [29; 41; 42]. The architecture of ant nests has been mostly studied in fungus-feeding ants such as _Paraponera_, _Pognomyrmex_, _Odontomachus_, _Campontotus_, and _Ectatomma_[43; 30]. However, the nest architecture of only a few species of the Pheidole genus has been studied, as is the case of _P. axyops_[44]. The _P. fallax_ nests analyzed presented a different architecture from those of _P. axyops_, which have irregular and vertically structured nests with convex chambers at the top, concave chambers at the bottom, and a mean depth of 223.77 cm [44]. _P. fallax_ nests are shallow, not exceeding 10 cm, and the chambers are irregular. However, some nests had a concave top chamber.