How the rhizosphere chemistry explains the effectiveness of radish in reclaiming legacy phosphorus compared to maize

Background and aims Around 50% of phosphorus (P) applied to tropical soils is not used by plants and become part of the legacy P. Some cover crops can extract this P. But how do they do this and 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 later and former bearing sorbed phosphate. The structure and composition of the root-soil 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 at 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 demonstrated a higher efficiency compared to maize. The total P uptake by radish from Al-bound P was 42% higher than that uptake 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, which the latter was 2.14-fold more than maize.

Radish uptakes greater Al-bound P than Fe-bound P. This insight could help in proper management of soils where P is predominantly bound to Al and significantly increases P use efficiency. Competing Interest Statement The authors have declared no competing interest. Abbreviations - µXRF - Microprobe X-ray fluorescence - µXANES - Microprobe X-ray absorption near edge structure - LCF - Linear Combination fitting - PCA - Principal Component Analysis - FTIR - Fourier-transform infrared spectroscopy - PUE - Phosphorus use efficiency - SEM - Scanning electron microscope - EDS - Energy dispersive X-ray spectroscopy