N₂O emissions, microbial community composition and genes expressions in soil amended with vermicomposts derived from different feedstocks

Vermicomposting is increasingly used to recycle organic wastes into highly-valued fertilizer. It is still unclear whether, and how, the feedstocks of vermicomposts affect soil N2O emissions. Thus, this study investigated the responses and relevant mechanisms of soil N2O emissions to the amendment of vermicomposts derived from the civil sludge and the cattle dung, using gas chromatography and microbial gene analysis. Compared to the cattle dung vermicompost, the civil sludge vermicompost increased soil N2O emission on an average of 14.5 times. This difference did not disappear when the concentrations of mineral N (NH4+ and NO3-) and metals (Zn and Fe) was adjusted to the same levels. This result indicated that the variations in mineral N and metal concentrations did not account for the difference in soil N2O emission between these two vermicomposts. The relative abundance of nirK-type denitrifying bacteria and the nirK gene expression were significantly higher in soil amended with the civil sludge vermicompost than those with the cattle dung vermicompost, suggesting that the promotion of nirK gene expression and the inhibition of N2O reduction potentially explained the higher N2O emissions under the civil sludge vermicompost amendment compared to the cattle dung vermicompost amendment. Controlling the nirK-type denitrifying bacteria and nirK gene expression at low levels and promoting N2O reduction will reduce soil N2O emission from vermicomposts.