Iron reduction promotes soil phosphorus dissolution and Chinese fir phosphorus uptake by altering the rhizosphere environment

In subtropical regions, frequent rainfall induces periodic anaerobic conditions in the soil which triggers redox reactions of iron (Fe) and alters the bioavailability of phosphorus (P). While the effects of rainfall events on these processes are well understood in agricultural ecosystems, their influence remains unclear on rhizosphere soil P availability and the P content of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) seedling tissues. Therefore, we conducted a greenhouse pot experiment, simulating a short-term (111 days) alternating anaerobic soil environment (95% field capacity) to investigate the dynamics of rhizosphere soil inorganic P in Chinese fir seedlings. The anaerobic treatments (denoted by N) were applied in three stages (S1, S2, S3) to seedlings of two varieties: ‘Yang 061’ (A—hygrophilous) and ‘Yang 020’ (B—drought-tolerant). A control group maintained at 70% field capacity was fully replicated for all treatments. Our results show that the anaerobic environment, coupled with increased root exudation of scopoletin, directly or indirectly promoted Fe reduction in the soil. Concurrently, available P concentrations increased by 42.8% and 66.3% in the AN1 and AN3 treatment groups, respectively, leading to increased total P content in seedling leaves and roots compared with the controls. Partial least-squares path models and random forest analyses indicated that the key drivers of inorganic P availability under anaerobic conditions differed between seedling varieties. For variety A, alpha and beta diversity, along with the dominant phylum (Proteobacteria), of the phosphate-solubilizing bacterial community were the dominant drivers, while dehydrogenase enzyme activity was an additional driver for variety B. These findings provide a scientific basis for understanding P cycling processes in the Chinese fir rhizosphere under changing rainfall regimes in subtropical areas.