Influence of long-term inorganic fertilization and straw incorporation influence on soil organic carbon (SOC) by altering C acquisition enzyme activity, active SOC fraction, soil aggregates, and microbial compositional and functional traits

Increasing soil organic carbon (SOC) stocks in agricultural systems is vital for mitigating climate change, improving soil quality, and enhancing grain production. However, the mechanisms by which fertilization practices influence SOC, particularly at the microscopic level, remain poorly understood. This study investigated the impacts of inorganic fertilizers in terms of nitrogen (N), phosphorus (P), and potassium (K), along with shallow (0–20 cm) straw incorporation (S), on soil properties, C acquisition (C-acq) enzyme activity, active SOC fractions, soil aggregates, and microbial carbon cycle functional traits, based on a 34-year field experiment conducted in the North China Plain. Six treatments of CK (control, without any fertilizer), NP (nitrogen + phosphorus fertilizers), NK (nitrogen + potassium fertilizers), PK (phosphorus + potassium fertilizers), NPK (nitrogen + phosphorus fertilizers + potassium), and NPKS (NPK + straw) were examined. Results indicated that NPK and NPKS treatments created favorable soil nutrient conditions, characterized by elevated levels of N, P, and K, along with reduced bulk density. Compared with NPK treatment, NPKS treatment led to significant increases in SOC (+38%) and active SOC fractions, including microbial biomass carbon (MBC, +17%), easily oxidizable carbon (EOC, +105%), and light fraction organic carbon (LFOC, +103%). Under NPK treatment, MBC and EOC increased by 43% and 40%, respectively, compared with CK. Both NPK and NPKS treatments enhanced macroaggregate formation, contributing to improved soil structure stability. Straw incorporation significantly boosted C-acq enzyme activity, whereas inorganic fertilizers had minimal impact. The abundance of functional genes involved in C-degradation were correlated significantly with soil C and N contents, C-acq enzyme activity, active SOC fractions, and macroaggregates, with higher levels observed under NP, NPK, and NPKS treatments than CK. Different C-fixation pathways responded variably to the measured soil traits, revealing no consistent trends across treatments. Structural equation modeling indicated that C-acq enzymes exerted a greater total effect on SOC than soil properties, active SOC fractions, and soil aggregates. Additionally, the abundance of functional genes related to C-degradation and methane metabolism played a more important role in SOC dynamic than those associated with C-fixation. In conclusion, NPK and NPKS treatments significantly enhanced SOC accumulation mainly by improving soil nutrient conditions, C-acq enzyme activity (notably for NPKS), active SOC fractions, macroaggregate formation, and the abundance of C-degradation genes. This study highlights the critical role of balanced inorganic fertilizers and straw incorporation in SOC accumulation, further elucidating the mechanisms influencing SOC dynamics.