Arsenic Fingerprint and Microbiome Reconstruction of Rhizosphere-Specific Nanophases in Flooded Paddy Soil

Abstract Ubiquitous arsenic (As) nanophases in soils are metastable due to the size effect and become readily available to rice under flooded conditions, making substantial contributions to the As pool. Here, the chemical characteristics of As nanophases in the rhizosphere as a hotspot for rice-soil-microbe interactions were fingerprinted at a single-particle resolution, revealing their chemical speciation and interactions with soil microbiota. The initial As particle characteristics (speciation and size) governed bioavailability through distinct transformation pathways mediated by particle-specific elemental associations. Specifically, nanoscale As sulfide and ferrihydrite with adsorbed As resulted in greater rice uptake than their microscale treatments, whereas nanoscale scorodite exhibited a comparable bioavailability. Analyses at the individual nanoparticle level revealed that 75–90% of As-bearing particles were associated with other metals regardless of the initial particle type. Within the As particles associated with Fe, the Fe-to-As mass ratios ranged from 27 to 41 despite low co-occurrence probabilities (<30%). The redistributions of As, Fe, and associated elements (e.g., Si/Mn/Zr) observed at the individual particle level aligned with the metabolic capacity of enriched rhizosphere microbiomes to drive biogeochemical cycling. Such microscale interactions might contribute to As transformation and plant resistance. By resolving metastable nanophase behavior at critical soil-plant interfaces, this work provides mechanistic insights into their dynamic speciation and bioavailability during key agricultural phases, advancing predictive frameworks for As mobility in agroecosystems.