## 4 Discussion  
In this study, the physicochemical properties and bacterial community of landfill soils were investigated. The total nitrogen content was low in all soil samples, probably because the landfill was mainly formed by construction waste, in which the biologically relevant nitrogen nutrient content was low [24]. The dynamic variations of groundwater led to divergence of subsurface soil properties and uneven distribution [25]. The subsurface soil ammonia nitrogen was higher than that in the topsoil, and the highest values appeared in samples W2-3, W3-2 and W4-3, which probably reflected the pathway of leachate migration. The ammonia nitrogen in samples W2D (9-15 m) was high (23.97 and 51.16 mg/kg), which indicated the subsurface soil in the ultra-deep layer had been contaminated due to seepage [26]. Previous study found that soil carbon and nitrogen ratios reflected the origin and components of the soil [16]. In this study, the C/N in topsoil was similar, indicating that the topsoil of samples W1-4 was homogeneous and closer in properties. The C/N in subsurface soil was higher than that in topsoil, probably because the subsurface soil biomass was lower. The bacterial diversity of topsoil and river water were higher than that of subsurface soil and groundwater, indicating that the growth of bacteria had been inhibited in the deep samples due to the harsh growth conditions and ammonia contamination [1]. The diversity of the groundwater samples was similar to that of the deep soil samples from 4-6 m, which laterally reflected the groundwater samples and deep soil samples taken from the same layer. Bacterial community structure results indicated that Proteobacteria was the dominant phylum in the soil samples and groundwater samples, ranging from 58.8% to 96.0%. Moreover, the percentage of species in Proteobacteria increased with depth, which indicated that the subsurface samples were not suitable for the growth of most phyla of bacteria. Similarly to Proteobacteria, the percentage of species in Firmicutes also increased with depth, probably because Firmicutes was often found in contaminated environments such as landfills [27; 28; 29]. At the genus level, the dominant genera in the topsoil were Others, accounting for 60%. The second most dominant genus was _Thibacillus_, which uses sulfide as substrates. In this study, the landfill gas was collected and discharged without treatment, resulting in the generation of malodorous gas, in which sulfide and nitride were important components. The sulfide in malodorous gas may have been the main reason for the high abundance of _Thibocallus_ in the topsoil. A large number of genera accounting for less than 0.5% of the total contributed to the high \(\alpha\)-diversity of topsoil. Similar results were found in river water, indicating that surface construction waste did not significantly change the diversity of bacteria. The unique dominant genus in river water was unclassified_family_ACK-M1, which was a characteristic genus of the local river. The absence of unclassified_family_ACK-M1 in soil samples and groundwater samples indicated that the river water did not recharge the groundwater. In the subsurface soil, the percentage of Others decreased with depth, probably because most of the genera accounting for small percentages of the total could not adapt to the survival conditions in the subsurface environment. However, the genus adapted to subsurface anaerobic oligotrophic conditions occupied the corresponding ecological niche and could become the dominant genus [6; 28]. Principal coordinates analysis results showed that there were large differences between the different soil layers while the community composition from the same layer was similar (Figure 3). The community composition between water and soil samples was separated on PCa2, because the dominant bacteria in soil and water were different [30]. Previous studies show that bacterial communities in soils are usually affected by variation in physicochemical factors [31; 32; 33].