Chemical nature of soil organic carbon under different long-term fertilization regimes is coupled with changes in the bacterial community composition in a Calcaric Fluvisol

Fertilization is an important factor influencing the chemical structure of soil organic carbon (SOC) and soil microbial communities; however, whether any connection exists between the two under different fertilization regimes remains unclear. Soils from a 27-year field experiment were used to explore potential associations between SOC functional groups and specific bacterial taxa, using quantitative multiple cross-polarization magic-angle spinning ¹³C nuclear magnetic resonance and 16S rRNA gene sequencing. Treatments included balanced fertilization with organic materials (OM) and with nitrogen (N), phosphorus (P), and potassium (K) mineral fertilizers (NPK); unbalanced fertilization without one of the major elements (NP, PK, or NK); and an unamended control. These treatments were divided into four distinct groups, namely OM, NPK, NP plus PK, and NK plus control, according to their bacterial community composition and SOC chemical structure. Soil total P, available P, and SOC contents were the major determinants of bacterial community composition after long-term fertilization. Compared to NPK, the OM treatment generated a higher aromatic C–O and OCH₃ and lower alkyl C and OCH abundance, which were associated with the enhanced abundance of members of the Acidobacteria subgroups 6 and 5, Cytophagaceae, Chitinophagaceae, and Bacillus sp.; NP plus PK treatments resulted in a higher OCH and lower aromatic C–C abundance, which showed a close association with the enrichment of unclassified Chloracidobacteria, Syntrophobacteraceae, and Anaerolineae and depletion of Bacillales; and NK plus control treatments resulted in a higher abundance of aromatic C–C, which was associated with the enhanced abundance of Bacillales. Our results indicate that different fertilization regimes changed the SOC chemical structure and bacterial community composition in different patterns. The results also suggest that fertilization-induced variations in SOC chemical structure were strongly associated with shifts in specific microbial taxa which, in turn, may be affected by changes in soil properties.