Legacy of rice roots as encoded in distinctive microsites of oxides, silicates, and organic matter

Rice (Oryza sativa) is usually grown under flooded conditions, leading to anoxic periods in the soil. Rice plants transport oxygen via aerenchyma from the atmosphere to the roots. Driven by O₂ release into the rhizosphere, radial gradients of ferric Fe and co-precipitated organic substances are formed. Our study aimed at elucidating the composition and spatial extension of those gradients. Air-dried soil aggregates from a paddy field were embedded in epoxy resin, cut, and polished to produce cross sections. Reflected-light microscopy was used to identify root channels. With nano-scale secondary ion mass spectrometry (NanoSIMS), we investigated transects from root channels into the soil matrix and detected¹² C⁻,¹⁶ O⁻,¹² C¹⁴ N⁻,²⁸ Si⁻,²⁷ Al¹⁶ O⁻, and⁵⁶ Fe¹⁶ O⁻ to distinguish between embedding resin, organic matter, oxides, and silicates. Image analyses reveal high occurrences of⁵⁶ Fe¹⁶ O⁻ within and in close proximity of oxide-encrusted root cells, followed by a thin layer with high occurrences of²⁷ Al¹⁶ O⁻ and¹² C¹⁴ N⁻ . In two of the three transects,²⁸ Si⁻ only occurs at distances larger than approximately 10 µm from the root surface. Thus, we can distinguish distinct zones: the inner zone is composed of oxide encrusted root cells and their fragments. A thin intermediate zone may occur around some roots and comprises (hydr)oxides and organic matter. This can be distinguished from a silicate-dominated outer zone, which reflects the transition from the rhizosphere to the bulk soil.