Biosolids amendment effects on nitrogen cycling gene expression by the soil prokaryotic community as revealed by metatranscriptomic analysis

Context Large quantities of treated sewage sludge (biosolids) are produced and beneficially applied to agricultural fields to improve soil fertility in many countries. Biosolids have extremely high concentrations of ammonium and organic matter that can be beneficial but also detrimental to the environment by promoting microbially-mediated reactions that contribute to eutrophication and greenhouse gas emission.Aims The hypothesis of the study was that high concentrations of ammonium and labile organic matter in biosolids would significantly affect nitrogen transformations and nitrogen-cycling gene expression by different members of the prokaryotic community in a biosolids-amended agricultural soil.Methods An organically-managed agricultural soil was amended with biosolids and monitored for changes in carbon dioxide and inorganic nitrogen species for 3weeks under laboratory conditions. Then, RNA was extracted and compared for nitrogen-cycling gene expression levels in biosolids-amended and unamended soil.Key results Biosolids amendment significantly increased ammonium concentration and decreased oxygen and nitrate concentrations in soil zones near biosolid particles, which coincided with significant changes in expression levels of genes for catabolic glutamate dehydrogenase, nitrification enzymes, denitrifying enzymes, and numerous other enzymes by different members of the prokaryotic community.Conclusions The application of biosolids to soil set in motion a dynamic organic nitrogen mineralisation–nitrification–denitrification cycle between the anaerobic biosolids zone and aerobic soil zone.Implications Biosolids-induced changes in nitrogen transformations by different members of the microbial community have implications on nitrogen availability/toxicity to nitrifying populations and plants, ammonium and nitrate in surface runoff, and nitrous oxide greenhouse gas emission from biosolids-amended soil.