Iron oxides promote physicochemical stabilization of carbon despite enhancing microbial activity in the rice rhizosphere

Rice rhizosphere soil is a hotspot of microbial activity and a complex interplay between soil abiotic properties, microbial community and organic carbon (C). The iron (Fe) plaque formation in the rice rhizosphere promotes Fe-bound organic C formation and increases microbial activity. Yet, the overall impact of Fe on C storage via physicochemical stabilization and microbial mineralization of rhizodeposits (rhizo-C) and soil organic C (SOC) in the rice rhizosphere remain unclear. We conducted a microcosm experiment using ¹³C-CO₂ pulse labeling to grow rice (Oryza sativa L.) with four levels of α-FeOOH addition (Control, Fe-10 %, Fe-20 %, Fe-40 % w/w of α-FeOOH per total Fe in soil). This study aimed to evaluate the impact of Fe oxides on rhizo-C mineralization, the rhizosphere priming effect, and Fe-OM formation. Microbial community composition and localization of enzyme activities were also quantified through 16S rRNA sequencing and zymography. The hotspot area, as being indicated by zymography, increased by 20-50% in the presence of Fe compared to the soil without Fe addition. Despite being a hotspot, strong coprecipitation of Fe-OM in the rhizosphere promoted C immobilisation. Fe-20 % and Fe-40 % resulted in a 41 % and 33 % decrease of rhizodeposits derived ¹³C-CO₂ emission and increased ¹³C stabilization mainly in 0.25-2 mm soil aggregates due to coprecipitation and aggregate formation with α-FeOOH. Moreover, Fe addition led to a dominance of Fe-oxidizing bacteria genera such as Pseudomonas, which fostered coprecipitation of Fe-OM formation. We highlight larger physicochemical stabilization of organic C by α-FeOOH addition despite raised hotspot area of microbial activity in the rice rhizosphere.