Effects of long-term fertilization on the diversity of bacterial mercuric reductase gene in a Chinese upland soil

Soil mercury (Hg) pollution has received considerable attention due to its neurotoxin effects and its potential risk to food safety. The microbial transformation of Hg plays a key role in reducing Hg toxicity by the mercuric reductase (MerA) conferred by genes arranged in the mer operon. This study investigated the effects of long-term fertilization on the diversity of bacterial mercuric reductase gene (merA), which specify the reduction of ionic Hg²⁺ to the volatile elemental form Hg⁰, in an agricultural soil with relatively high Hg content. The soil samples were collected from different treatments, including control without fertilizer (CK), fertilizer nitrogen (N), combined fertilizers (NPK) of N, phosphorus (P) and potassium (K), and NPK plus organic manure (NPK + OM). The merA gene diversity patterns were analyzed based on the merA clone libraries and sequencing measurements. Results showed that the merA gene diversity was influenced by soil variables depending on the fertilization practices. In particular, NH₄⁺ and NO₃⁻ contents had strong effect on the merA gene diversity pattern both in the N and NPK treatments, whereas the merA gene diversity pattern in NPK + OM treatment was distinctly influenced by the contents of organic matter, available P and K. These results suggested that long-term fertilization had significant influences on merA gene diversity, which could be helpful to understand the Hg reduction process and potentially serve microbial remediation of Hg contaminated soil.