Effects of co-application chemical and organic fertilizers on microbial residues and their contribution to soil organic carbon accumulation

Background Fertilization strategies in agricultural ecosystems significantly influence soil organic carbon (SOC) dynamics, and integrated chemical and organic fertilization may enhance short-term SOC accumulation. However, the mechanisms, particularly the contribution of microbial residues to SOC dynamics during the crop growth period, remain unclear. Methods In this study, we evaluated four fertilization treatments in tobacco-planting field under a randomized block design with three blocks: (1) no fertilizer (CK), (2) chemical fertilizer (CF), (3) chemical fertilizer with corn straw amendment (CFS), and (4) chemical fertilizer with manure amendment (CFM). Soil samples (0–20 cm) were collected at six stages of the tobacco-growing period, and were analyzed for SOC, soil physicochemical properties, and amino sugar biomarkers (MurA, GalN, and GluN) to calculate microbial residue-C fractions. Results Fertilization significantly affected soil nutrient status, amino sugar pools, microbial residue-C fractions, and SOC over time compared with CK and CF, the two organic-amended treatments were associated with higher SOC concentrations, and CFM showed the strongest short-term SOC response (peaked at 10.30 ± 0.22 g kg −1 at 30 d). Amino sugar concentrations and microbial residue-C fractions were generally higher under CFM and CFS than under CF and CK, especially during the middle and late stages. Correlation analysis showed that SOC was positively correlated with BRC and MurA under CFM (both r = 0.58, P 0.05). The merged piecewise linear mixed-effects SEM indicated that fertilization treatment, time, and their interaction all had significant positive effects on soil physicochemical properties, microbial residues, and SOC. Conclusion Integrated chemical-organic fertilization, particularly CFM, favored short-term SOC accumulation in tobacco-planting soil. Amino sugar-derived microbial residues were also enhanced under organic-amended fertilization, and bacterial residues were more closely associated with SOC than fungal residues during the observed period. The merged piecewise linear mixed-effects SEM indicated that short-term SOC dynamics were driven mainly by fertilization regime and temporal variation, providing region-specific evidence for optimizing fertilization strategies in tobacco-planting red soils of Yunnan.