Long-term fertilisation reveals close associations between soil organic carbon composition and microbial traits at aggregate scales

Fertilisation plays key roles in soil organic carbon (SOC) formation and stabilization by regulating a range of microbial traits. However, little is known about the relationships between SOC composition and microbial traits under long-term fertilisation at aggregate scales. Here, we selected four long-term fertilisation field experiments in China to evaluate the potential associations between SOC composition and microbial traits within aggregates. The four experiments have been treated for more than 25 years with inorganic nitrogen (N), phosphorus (P) and potassium (K) fertilisers (NPK), organic manure (M), and NPK plus M (NPKM). After aggregate isolation, SOC physical fractions including free particulate organic carbon (fPOC), occluded particulate organic carbon (oPOC) and mineral organic carbon (MOC) was measured via density fractionation. SOC chemical structure was determined by 13C nuclear magnetic resonance, and the bacterial community composition was analysed using amplicon sequencing of the 16S rRNA genes. The results showed that fertilisation increased the contents of free particulate organic carbon (fPOC) and occluded particulate organic carbon (oPOC) while decreasing the relative abundance of aromatic C in macroaggregates. Irrespective of fertilisation regime, the relative abundance of cellulose-degrading genes (cbhI and GH48 genes) was higher in the clay and silt fractions than in the macro- and microaggregates. Structural equation modeling indicated that SOC chemical structure was associated with pH-driving shifts in bacterial community composition, and its physical fractions were associated with soil nutrients-induced (mainly total N and P) changes in cellulose-degrading genes and specific taxa in macroaggregates. Finally, we conclude that fertilisation changed soil pH and total N, which were the major drivers affecting the SOC physical fractions and chemical structure, and the effects were caused by altering the bacterial community composition and fungal genes involved in C cycling within soil aggregates following long-term fertilisation.