Biogeochemical processes and arsenic enrichment around rice roots in paddy soil: results from micro-focused X-ray spectroscopy

The spatial distribution and speciation of iron (Fe), manganese (Mn) and arsenic (As) around rice roots grown in an As-affected paddy field in Bangladesh were investigated on soil sampled after rice harvest. Synchrotron micro-X-ray fluorescence spectrometry on soil thin sections revealed that roots influence soil Fe, Mn and As distribution up to 1 mm away from the root―soil interface. Around thick roots (diameter around 500 μm), Mn was concentrated in discrete enrichments close to the root surface without associated As, whereas concentric Fe accumulations formed farther away and were closely correlated with As accumulations. Near thin roots (diameter < 100 μm), in contrast, a pronounced enrichment of Fe and As next to the root surface and a lack of Mn enrichments was observed. X-ray absorption fine structure spectroscopy suggested that (i) accumulated Fe was mainly contained in a two-line ferrihydrite-like phase, (ii) associated As was mostly As(V) and (iii) Mn enrichments consisted of Mn(III/IV) oxyhydroxides. The distinct enrichment patterns can be related to the extent of O2 release from primary and lateral rice roots and the thermodynamics and kinetics of Fe, Mn and As redox transformations. Our results suggest that in addition to Fe(III) plaque at the root surface, element accumulation and speciation in the surrounding rhizosphere soil must be taken into account when addressing the transfer of nutrients or contaminants into rice roots.