Effects of nickel and cobalt on methane production and methanogen abundance and diversity in paddy soil.

Background: Paddies are an important anthropogenic source of methane emissions to the atmosphere, and they are impacted by heavy metal pollution. Nickel (Ni) and cobalt (Co) pollution might either enhance or mitigate CH4 emission from paddy soils due to the total amounts of metals, bioavailability and functional microbial activity and composition.

Methods: An incubation experiment was conducted, and different Ni and Co concentrations were added to test the effects of trace metals on methane production in paddy soil. The archaea community structure and the abundance of methanogen functional groups in the paddy soil with added Ni and Co were detected using high-throughput sequencing and quantitative PCR based on the 16S rRNA and mcrA (methyl coenzyme M reductase) genes, respectively.

Results: The highest methane production rate was 561 mg CH4 kg-1 dry soil d-1 with the addition of 50 mg kg-1 Ni and 684 mg CH4 kg-1 dry soil d-1 with the addition of 25 mg kg-1 Co. Accordingly, the mcr A gene was most abundant in the 50 mg kg-1 Ni addition (3.1 × 106  ± 0.5 × 106 copies g-1 dry soil). The lowest mcr A gene abundance was detected in the 500 mg kg-1 Co addition (9.2× 105  ±  0.4 × 105 copies g-1 dry soil). The dominant methanogens were Methanobacterium, Methanosarcina, Methanocella, Methanomassiliicoccus, Bathyarchaeota, and Rice Cluster I (RC-I), and the relative abundances of these groups were higher than 1% in the Ni and Co treatments. Additionally, the archaeal compositions differed significantly in the soils with various Ni and Co additions. The most abundant Methanococcus spp. represented 51.3% of the composition in the 50 mg kg-1 Ni addition, which was significantly higher than that of the control (12.9% to 17.5%).

Discussion: Our results indicated that the contamination of soil by Ni and Co significantly affected total methanogens abundance and specific methanogen functional groups. Ni and Co additions to paddy soil promoted methanogenic activity at low concentrations, while they had inhibitory effects at high concentrations. Because paddy soils largely contribute to methane emissions and are increasingly exposed to heavy metal pollution, our results show that future assessments of greenhouse gas flux from paddy soils should take into account the effects of pollution by Ni and Co.