Long-term metal pollution shifts microbial functional profiles of nitrification and denitrification in agricultural soils

The increasing contamination of heavy metals in agricultural soils and its impact on the nitrogen (N) cycle and N use efficiency have attracted considerable attention in recent years. In this study, agricultural soils neighboring the Dabaoshan copper mining area (DBS) and Qingyuan electronic-waste recycling area (QY), in Guangdong, China, were sampled to study the interaction between heavy metals and nitrification/denitrification processes, especially the related microbial functional profiles. Results showed that the contamination of heavy metals affected nitrifiers and denitrifiers differently. The potential nitrification activity was about four times lower in metal-polluted soils compared with the unpolluted ones, with a significant decrease in the abundance of amoA and nxrB (p < 0.05) in the polluted samples. On the other hand, the potential denitrification activity was more metal-resistant, which attributed to its complex species composition as shown by a slightly higher α-diversity index, and was slightly higher (p > 0.05) in the polluted samples. Among the five denitrifying genes tested, nosZ gene had the highest increase and the nirK gene the most decrease in numbers and in the polluted soils. The metal-polluted soils had fewer correlations among N functional genes based on the co-occurrence network analysis. In addition, the core taxa of the whole bacterial community changed from copiotrophic to oligotrophic bacteria in the presence of heavy metals. Mantel test indicated that heavy metals were the dominant factors determining N-related genes while the bacterial community composition was due to a combination of heavy metal presence and soil properties such as TOC, NO₂⁻, and pH. It is concluded that long-term heavy metals pollution potentially affected nitrifiers and denitrifiers differently as indicated by the shift in N functional genes and the change in nitrification/denitrification processes.