Organic–Inorganic and Microbial Fertilisation Under Continuous Sorghum Cropping: Grain Yield, Soil Properties, and Rhizosphere Microbial Community Responses

Continuous sorghum cropping causes yield decline in semi-arid regions. In a two-year field experiment conducted on plots previously cropped with sorghum for two years, four fertilisation treatments—no fertiliser (SCK), chemical fertiliser (SCF), organic–inorganic compound fertiliser (SOIF) and compound microbial fertiliser (SCMF; SOIF supplemented with Bacillus subtilis and B. licheniformis)—were compared in terms of grain yield, soil physicochemical properties, potential enzyme activities and rhizosphere bacterial and fungal communities. Year, treatment and year × treatment effects were tested using mixed models that accounted for repeated measurements of the same plots. The complete SCMF formulation was associated with the highest grain yield (10,108.13 kg ha−1, in 2025), exceeding those under SCK, SCF, and SOIF by 42.95%, 11.47%, and 3.59%, respectively. The SCMF-SOIF comparison, in which N, P2O5, K2O and organic-matter inputs were identical, isolates the microbial inoculants; its yield increment was small and not significant in either year (4.27% and 3.59%), so an inoculant-specific yield benefit was not demonstrated. Compared with SOIF, SCMF had higher alkaline phosphatase (ALP) activity in both years and higher sucrase (SUS) activity (+36.80%) and available nitrogen (AN; +21.11%) in 2025. In 2025, SOIF and SCMF reduced the topsoil bulk density relative to SCK and SCF, and SCMF shifted the aggregate-size distribution towards the 2–0.25-mm fraction (+47.45–112.62%). Fertilisation altered the composition, but not the alpha diversity, of rhizosphere communities; the treatment effect was larger and more independent of multivariate dispersion for fungi (R2 = 0.167, p = 0.009) than for bacteria (R2 = 0.113, p = 0.049). The fungal response represents an overall treatment-associated compositional difference with weaker statistical evidence in the second year, because treatment differences were significant only in 2024 when each year was tested separately. Nitrosospira, a genus containing members previously associated with ammonia oxidation, was consistently enriched under SCMF, although nitrification was not measured. Overall, the yield benefit was attributable mainly to organic–inorganic fertilisation, whereas inoculant-associated differences were found in potential enzyme activities and fungal community composition rather than in grain yield.