Microbial Activation Promotes Mineral-Associated Organic Matter Formation under Organo-Mineral Fertilization in Semi-Arid Irrigated Orchards of Central Asia

Semi-arid irrigated agroecosystems are increasingly recognized as climatically vulnerable yet strategically important for soil organic carbon (SOC) stabilization. Light chestnut soils of southeastern Kazakhstan (1.3% humus; alkaline reaction) are characterized by low microbial activity and limited intrinsic carbon protection capacity, making them prone to SOC depletion under intensive management.

We hypothesized that organo-mineral fertilization stimulates microbial carbon processing and enhances the formation of mineral-associated organic matter (MAOM) through microbial biomass turnover and necromass stabilization.

A three-year field experiment (2021–2023) was conducted in irrigated apple orchards (Malus domestica Borkh., cv. Golden Delicious). Mineral fertilization (N90P90K90) was compared with integrated organo-mineral treatments combining NPK with biohumus, humic substances, or microbial biopreparations. Microbial functional groups (total bacteria, Pseudomonas spp., Azotobacter spp., and fungi) were quantified to evaluate shifts in carbon transformation pathways.

Organo-mineral treatments increased total bacterial abundance by 2–2.5 times and Pseudomonas spp. populations by up to four times relative to mineral-only fertilization. Yield increased by up to 64% without reduction in soluble sugar concentration, suggesting improved carbon-use efficiency rather than biomass dilution.

A conceptual carbon-flow framework indicates enhanced labile carbon turnover, microbial biomass production, necromass accumulation, and subsequent stabilization within MAOM pools, consistent with the Microbial Carbon Pump paradigm.

A conservative stabilization estimate suggests additional sequestration of approximately 0.48 t C ha⁻¹ yr⁻¹ (≈1.7–1.8 t CO₂-eq ha⁻¹ yr⁻¹). These findings demonstrate that semi-arid irrigated orchards can function as active microbial-driven carbon stabilization systems under integrated nutrient management.