Stage-specific phosphorus mobilization enhances phosphorus uptake in relay-intercropped soybean

Intercropping systems exploit the phosphorus (P)-activating ability of leguminous plants to enhance P acquisition. However, the specific P mobilization strategies used by leguminous plants, such as soybean (Glycine max L.), at different growth stages remain unclear. This study examined the P mobilization strategies of soybeans across developmental stages within a maize-soybean relay intercropping system. A 2-year field experiment was conducted to assess variations in P accumulation, root growth, organic acid (OA) secretion, acid phosphatase (APase) activity, and soil P fraction dynamics from the seedling stage to full pod development. Intercropped soybean mobilized P through growth-stage-specific strategies. During the beginning flowering (R1) stage, root OA secretion in intercropped soybeans peaked. Under both P0 and P20 application conditions, the average secretion level was approximately 40 % higher than that in monocropped soybeans. This OA secretion advantage persisted until the beginning pod (R3) stage, after which, OA levels became comparable with those in monocropped plants. At R3, regardless of P application, APase activity in intercropped soybean peaked and was significantly higher than that in monocropped soybeans. This advantage over monocropping persisted until the beginning seed (R5) stage. Soil available P (AP) content at R3 was significantly higher in intercropping and remained elevated through R5, supporting P uptake during the pod-filling stage. Correlation analysis revealed that soil AP content at R3 was highly significantly correlated with OA secretion at R1 and APase activity at R3. Further structural equation modeling revealed that early OA secretion released stable P pools, while later APase activity maximized P availability from organic sources. This process aligned P mobilization with the peak growth demands of soybeans. These findings provide insights into how relay intercropping synchronizes P mobilization with the developmental requirements of soybeans.