Degradation of soil aggregate stability is associated with phosphorus limitation in successive eucalyptus plantations

Intensively managed Eucalyptus plantations are critical timber supply, yet their long-term sustainability is threatened by declining productivity linked to phosphorus (P) limitation. This study aimed to reveal and quantify the mechanisms underlying this process using a chronosequence of first- to third-generation plantations in subtropical China, integrating analyses of soil aggregate stability, sequential P fractionation, active Al/Fe oxides, and microbial functional guild dynamics. We found that successive planting was associated with degradation of aggregate stability and severe soil acidification. This physicochemical deterioration coincided with shifts in P-cycling-related microbial indicators, including declines in arbuscular mycorrhizal fungi, phosphate-solubilizing bacteria, and acid phosphatase activity. Concurrently, P speciation shifted systematically, with the contribution of bioavailable P pools decreasing by up to 20.6% while recalcitrant pools increased by 17.84%. Path analysis revealed that escalating chemical stress (soil acidification and Al/Fe activation) showed the strongest negative association with P availability (path coefficient β = −0.465), while aggregate stability showed the strongest positive association (β = 0.384). Generation-specific models indicated that the P transformation network shifted from a complex, multi-pathway configuration in the first generation to a constrained system with fewer supportive linkages by the third generation. We conclude that the decline in P availability is associated with coupled physical, chemical, and biological changes, wherein physicochemical deterioration coincides with reduced biological P-mobilizing capacity. These findings provide a process-based framework for diagnosing soil degradation and developing integrated management strategies for intensively managed plantations.