Insights into the effects of heavy metal pressure driven by long-term treated wastewater irrigation on bacterial communities and nitrogen-transforming genes along vertical soil profiles.

Irrigation with treated wastewater (TWW) influences soil ecological function due to the accumulation of heavy metals (HMs) and nutrients in soils. However, the interaction between HMs and microbial processes in TWW-irrigated soil has not been fully explored. We investigated the effect of HMs on bacterial communities and nitrogen-transforming (N-transforming) genes along vertical soil profiles irrigated with domestic TWW (DTWW) and industrial TWW (ITWW) for more than 30 years. Results indicate that long-term TWW irrigation reshaped bacterial community structure and composition. Irrigation with ITWW led to increased accumulation of Cd, Cr, Cu, Pb, Zn, and Ni in soils than DTWW. Accumulation of inorganic N, soil organic carbon, and HMs in topsoil irrigated with ITWW contributed to the activities of Micrococcaceae. The effect of the activation of nutrient factors on Bacillus, which was the dominant species in DTWW-irrigated soils, was greater than that of HMs. HM pressure driven by ITWW irrigation changed the vertical distribution of N-transforming functional genes, increasing the abundance of amoA gene and decreasing that of nifH through soil depth. ITWW irrigation enhanced the denitrification capacity in topsoil; ammonia-oxidizing capacity in deeper soil was increased after long-term irrigation with DTWW and ITWW, suggesting a potential risk of nitrogen loss.