Vermicompost and Organic Manure Interactions: Effects on Heavy Metal Concentrations, Nitrification Activity, Comammox Nitrospira inopinata, and Archaea/Bacteria

Abstract Vermicompost is a substantial source of nutrients, promotes soil fertility, and maintains or increases soil organic matter levels. Potentially toxic elements (PTEs) in vermicompost impact on nitrification activity. However, it is yet unknown how vermicompost affects nitrifying bacteria and archaea, comammox Nitrospira inopinata (complete ammonia oxidizers), net nitrification rates (NNRs), and PTEs. The effects of vermicompost application on NNRs, potential nitrification rates (NPs), PTEs, and the abundances of comammox N. inopinata bacteria, nitrite-oxidizing bacteria (NOB) and ammonia-oxidizing bacteria (AOB)/archaea (AOA) were studied. NNRs and NPs were significantly higher (p<0.05) in fresh cow-dung vermicompost (stored for 40 days) as compared with other organic manure. The level of PTEs (Cu2+, Fe2+, Pb2+, Cd2+, and Zn2+) was significantly lower (p<0.05)) in vermicompost as compared with compost of waste material with Trichoderma and cow-dung. Comammox N. inopinata, NOB, AOB, and AOA were significantly higher (p<0.05) in stored cow-dung vermicompost (more than one year) as compared with other organic manure. The results of the Scatter plot matrix analysis suggested that Fe2+, total nitrogen (TN), soil organic carbon (SOC), and total carbon (TC) were linearly correlated (p<0.001) with NNRs and NPs in vermicompost and organic manure. Similarly, comammox N. inopinata bacteria, NOB, AOB, and AOA were linearly correlated (p<0.001) with NNR and NP. It may be concluded that the influence of vermicompost, which played a great role in PTEs concentrations reduction, increased chemical and biological properties, increased the growth rate of nitrifying bacteria/ archaea and the nitrogen cycle.