Organic and inorganic amendments did not affect microbial community composition in the bulk soil differently but did change the relative abundance of selected taxa

Deep placement of organic amendments such as poultry litter and legume residues has been developed as a technique to ameliorate sodic clay subsoils in the south‐east Australian cropping region. It has been suggested that the soil microbial community plays a key role in this process of subsoil amelioration. A controlled environment experiment using repacked soil units was conducted to investigate changes in the microbial communities of subsoil treated with organic and inorganic amendments. Poultry litter, inorganic nutrients and wheat straw plus inorganic nutrients with equivalent total nutrient content (460 kg N ha⁻¹, 210 kg P ha⁻¹ and 330 kg K ha⁻¹) were mixed into a central band and incubated for 21 days, followed by 126 days with wheat plants. Changes to bacterial and fungal communities in the bulk subsoil, 2–7 cm from the amended band, were assessed using next‐generation amplicon sequencing of the 16S and ITS rRNA genes. There were small but significant (p < 0.05) changes in bacterial and fungal community structure of the amended treatments, relative to the unamended control, which were linked to the movement of solutes away from the amended band. The three amendments did not affect microbial community composition differently in the bulk soil, although there were changes in the relative abundance of selected taxa. Our results suggest that changes in soil microbial communities in the bulk subsoil in this experimental system largely depended on movement of the soil solution through the profile. HIGHLIGHTS: We characterized effect of amendments on microbial communities in the soil beyond the detritusphere. Microbial community of bulk soil was different in amended treatments from the unamended control. Amendments did not affect community composition differently but did change relative abundances of certain taxa. Changes in microbial community structure were due to movement of soil solution away from the amendment.