Objectives This study aimed to investigate the integrated effects of different organic fertilizer substitution ratios for chemical nitrogen (N) fertilizer on soil organic carbon (SOC) mineralization, aggregate structure, and microbial community composition in rubber plantation soils of Hainan, China. The underlying mechanisms were explored to identify the optimal substitution ratio, thereby providing a scientific basis for improving soil quality and enhancing carbon (C) sequestration in rubber plantations.

Methods A field experiment was conducted in a rubber plantation in Hainan, China. Six treatments were implemented: a nitrogen-free control receiving only chemical phosphorus (P) and potassium (K) fertilizers (CK), chemical NPK fertilizer application (NPK), and four treatments with organic fertilizer substituting chemical N at ratios of 25%, 50%, 75%, and 100% (denoted as M25, M50, M75, and M100, respectively). Soil samples (0−20 cm depth) were collected from the 0−10 cm and 10−20 cm layers. An incubation experiment was conducted to determine SOC mineralization. The SOC mineralization potential (C₀), mineralization rate, and mineralization ratio (C₀/SOC) were calculated using a first-order kinetic model. Wet sieving was employed to separate different aggregate-size fractions, and their SOC contents were measured. Microbial community composition was determined by high-throughput sequencing.

Results 1) Compared with the NPK treatment, organic fertilizer substitution increased cumulative SOC mineralization and C₀, while significantly reducing the C₀/SOC ratio. In the 0−20 cm layer under the M50 treatment, cumulative SOC mineralization ranged from 1823.9 to 1950.8 mg/kg, which was 4.2%−6.6% higher than that under the CK treatment. The M50 treatment significantly increased C₀ by 8.7% in the 0−10 cm layer and decreased C₀/SOC by 20.3%−21.5% in the 0−20 cm layer compared with the NPK treatment. 2) Macroaggregates (>0.25 mm) accounted for 57.4%−64.5% of the bulk soil. In the 0−10 cm layer, the mean weight diameter (MWD) and geometric mean diameter (GMD) under the M50 treatment were 0.79 mm and 0.51 mm, respectively, which were 7.5% and 17.6% higher than those under the NPK treatment. Moreover, the M50 treatment significantly enhanced SOC enrichment in macroaggregates by 129.0% compared with the NPK treatment. 3) Regarding bacterial communities, the M50 treatment increased the relative abundance of Firmicutes in the 0−10 cm soil layer by 193.5% compared with the NPK treatment. Except for the M75 treatment, the relative abundance of Bacteroidota showed an increasing trend with higher organic fertilizer substitution ratios (54.6%−80.0% higher than the NPK treatment). In fungal communities, compared with the CK treatment, the NPK treatment reduced the relative abundance of Ascomycota by 25%−28.6%. The four organic fertilizer substitution treatments increased the relative abundance of Ascomycota by 18.2%−25.0% compared with the NPK treatment. Correlation analysis indicated that cumulative SOC mineralization was significantly positively correlated with the content of water-stable aggregates >0.25 mm (R₀.₂₅), MWD, GMD, and SOC content, but significantly negatively correlated with C₀/SOC. The abundance of Ascomycota was significantly positively correlated with R₀.₂₅, MWD, and GMD (P < 0.05).

Conclusions Organic fertilizer substitution for chemical fertilizer enhanced soil C sequestration and aggregate stability by regulating microbial community composition, promoting macroaggregate formation, and increasing SOC accumulation within macroaggregates. Replacing 50% of chemical N fertilizer with organic fertilizer (M50) represents the optimal strategy for the sustainable management of rubber plantation soils in Hainan, China.