Levels of heavy metal concentrations and their effect on net nitrification rates and nitrifying archaea/bacteria in paddy soils of Bangladesh

Abstract

Heavy metal concentrations (HMCs) may alter nitrification activity in soil. However, the levels of HMCs in paddy soil and the relations among HMCs, net nitrification rates, and nitrifying archaea/bacteria in paddy soils of Bangladesh remain unknown. The goal of this study was to determine the effect of HMCs on net nitrification rates and ammonia-oxidizing archaea (AOA) and bacteria (AOB), and nitrite-oxidizing bacteria (NOB) in paddy soils. HMCs and net nitrification rates were assayed through an atomic absorption spectrophotometer (AAS) and mass spectrophotometer (MS), respectively, and the abundance of AOA, AOB, and NOB was quantified by q-PCR. According to the contamination factor (Cf), concentrations of Cd were found to be a very severe pollutant in all soils. On the other hand, concentrations of Fe and Cu were found to be moderately contaminated and slightly polluted in paddy soils as compared to control soils. The net nitrification rates (8.67 to 12.75 mg kg−1 h−1) and the abundance of AOA, AOB, and NOB were significantly higher (p < 0.05) in the paddy soils than in the control soils. AOB, AOA, and NOB were correlated positively with net nitrification rates. The relations between net nitrification rates and HMCs (Fe, Pb, and Zn) were correlated significantly and positively (p < 0.05), but the concentration of Cu was correlated negatively with net nitrification rates. To the best of our knowledge, this is the first report that has investigated the effects of HMCs on nitrifying bacteria in paddy soils, which will change the scenario of nitrification in Bangladesh greatly.