How the Rhizosphere Chemistry Explains the Effectiveness of Radish (Raphanus sativus L.) in Reclaiming Legacy Phosphorus Compared to Maize (Zea mays L.)

Around 50% of phosphorus (P) applied to tropical soils is not used by plants and becomes part of the legacy P. However, certain cover crops can access this P pool. But how do they extract this P, which form do they extract most? Radish (Raphanus sativus L.) and maize (Zea mays L.) were planted in rhizoboxes and tubes containing a mixture of sand, kaolinite, hematite, and boehmite, the latter and former bearing sorbed phosphate. The structure and composition of the root-substrate interface were determined by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). Additionally, synchrotron-based microprobe X-ray fluorescence (µXRF) was employed to assess the spatial distribution of Al, Fe, and P. The P speciation in the rhizosphere was evaluated using microprobe X-ray absorption near-edge structure (µXANES). The chemical images revealed that both plants depleted more of the Al-bound P than Fe-bound P, with radish demonstrating a higher efficiency compared to maize. The total P uptake by radish from Al-bound P was 42% higher than that from Fe-bound P. Additionally, radish absorbed 34% to 90% more total P compared to maize, indicating a significant difference between the two crops. The superior capacity exhibited by radish seems to be connected to organic acids and total carbon exudation, with the latter being 2.14-fold more than maize. Radish outperforms maize, extracting greater Al-bound P than Fe-bound P. This insight could support the effective management of soils where P is predominantly bound to Al, enhancing P use efficiency.