Out of sight: Profiling soil characteristics, nutrients and bacterial communities affected by organic amendments down to one meter in a long-term maize experiment

Common soil characteristics, nutrients and microbial activity at deeper soil depths are topics seldom covered in agricultural studies. Biogeochemical cycles in deep soils are not yet fully understood. This study investigates the effect of different mineral and organic fertilisation on soil organic matter dynamics, nutrients and bacterial community composition in the first meter of the soil profiles in the long-term maize cropping system experiment Tetto Frati, near the Po River in northern Italy. The following treatments have been applied since 1992: 1) crop residue removal (CRR), 2) crop residue incorporation (CRI), 3) crop residue removal with bovine slurry fertilisation (SLU), 4) crop residue removal with farmyard manure fertilisation (FYM). A total of 250 kg N ha−1 were applied annually as mineral fertiliser in the first two and as organic fertilizer in the latter two treatments. Soil organic carbon (SOC) was significantly higher in the treatments with organic amendments (CRI, SLU and FYM) compared to CRR in 0–25 cm (11.1, 11.6, 14.7 vs. 9.8 g kg−1, respectively), but not in the deeper soil. At 75–100 cm soil depth, SLU and FYM had the highest potential N mineralisation. Bacterial diversity decreased down the soil profile much less than microbial biomass. Incorporation of crop residues alone showed no positive effects on either biomass or diversity, whereas fertilisation by FYM instead of mineral fertilizer did. Bacterial community composition showed depth-related shifts: Proteobacteria and Actinobacteria dominated the topsoil, whereas Chloroflexi, Nitrospira and Thermotogae were relatively more abundant deeper in the soil profile. Although the main factor determining soil bacterial community composition in the entire dataset was soil depth, both the size and diversity of bacterial community, as well as several discriminating taxa, were affected by organic N fertilisation down to 1 m depth. This calls for continued efforts to study the deeper soil depths in the numerous long-term field experiments, where mostly topsoils are currently studied in detail.