Effects of different concentrations and types of Cu and Pb on soil N-fixing bacterial communities in the wheat rhizosphere

Abstract

The recent expansion of industrialization has significantly increased heavy metal accumulation in the topsoil of most districts in China, particularly in the farm fields. Heavy metal concentration in the soil determines its chemical properties – especially the pH and nitrogen (N) content. Since plants are dependent on soil N-fixing bacterial communities (SNB) for the availability of this nutrient, it is essential to assess the impact of heavy metals on SNB composition. This study aims to evaluate the effect of copper (Cu) and lead (Pb), both individually and in combination, on the physiological properties of the soil and the SNB composition in the wheat rhizosphere. The number of SNB species and species richness decreased significantly following heavy metal exposure, likely due to considerable soil acidification, which further increased heavy metal accumulation and dissolution. Pb exerted greater toxic effects on SNB composition compared to Cu due to higher acidification. However, alpha diversity of SNB was not significantly affected by either heavy metal, even when combined, indicating that heavy metals do not always synergistically act on soil bacterial communities. Some SNB taxa, mainly c__Alphaproteobacteria, o__Sphingomonadales, f__Desulfarculaceae, o__Micrococcales and f__Clostridiales Family XVII Incertae Sedis, showed significant alterations under high concentrations of Cu and/or Pb. This is likely due to the selective pressure exerted by the heavy metals on different SNB which increases the abundance of some and decreases that of others. Taken together, soil pH is an important indicator of heavy metal-induced shift in SNB composition due to the altered uptake and utilization of nutrients. The findings offer new insights into the synergistic effects of Cu and Pb in the soil, which can have long-term effects on crop yield.