Soil Organic Carbon Fractions, Latex Biochemical Composition, and Yield of Rubber Trees under Bacillus-Based Bio-Organic and Chemical Fertilization

Extensive rubber cultivation using long-term chemical fertilization (CF) has accumulated negative consequences for soil ecosystems, particularly in replanting areas, potentially resulting in reduced physiological metabolism and yield of rubber trees. Bio-organic fertilizer, a compost of organic manures combined with plant-beneficial microbes, may influence SOC dynamics and soil fertility, thereby supporting plant nutrient uptake, tree metabolism, and crop production. This study quantified SOC fractions in 15- and 25-year-old rubber plantations treated with Bacillus-based bio-organic fertilizer (BBOF) or CF to evaluate their SOC fraction responses to different fertilizer regimes. Latex biochemical composition and yield were investigated to evaluate the physiological responses of rubber trees to these fertilizer treatments. In the 15-year-old plantation, BBOF significantly increased topsoil F1 (p = 0.023) but decreased subsoil F1 (p = 0.036), whereas latex biochemical parameters and yield did not differ significantly between fertilizer treatments. In the 25-year-old plantation, F1 was numerically higher under BBOF but did not differ significantly from CF, while topsoil F2 was significantly lower (p < 0.001). Taken together, the contrasting F1 and F2 responses indicated redistribution among oxidizable SOC fractions rather than a consistent increase in the combined labile SOC pool under BBOF. BBOF significantly increased latex Suc content and yield and decreased latex Mg content in the 25-year-old plantation. Overall, BBOF was associated with changes in SOC fractions and selected latex physiological parameters, together with higher latex yield, particularly in the 25-year-old plantation, suggesting its potential as a complementary fertilization strategy for established rubber plantations.