Carbon stabilization by iron plaque on rice roots: The role of oxygen loss

Abstract

Radial oxygen loss (ROL) from rice roots plays a crucial role in iron plaque formation in paddy soils, which are regional hotspots for carbon accumulation and critical for climate change. While the role of soil oxygenation around rice roots is well-recognized for root functioning and nutrient uptake, the ROL effects on microbial communities and organic carbon (OC) stabilization remain largely unexplored. To address this gap, we combined soil zymography, ¹⁴C imaging, and planar optodes to investigate how ROL influences iron plaque formation and OC stabilization. We cultivated three rice varieties (Yangdao6, Nongken57, and Huanghuazhan) with contrasting ROL intensities. Among them, Huanghuazhan, a rice cultivar with greater aerenchyma, had higher oxygen loss than Yangdao6 and Nongken57. The increased ROL raised the OC content in the rhizosphere by trapping it within the iron plaque, with specific values of 1.26 ± 0.25 mg C g⁻¹ root for Yangdao6, 3.22 ± 0.19 mg C g⁻¹ for Nongken57, and 4.36 ± 0.34 mg C g⁻¹ for Huanghuazhan. This increased iron plaque-trapped OC, in turn, raised Fe-bound OC content in the rhizosphere. The ROL increase reshaped rhizosphere microbiomes, driving synergistic proliferation of iron-oxidizing and iron-reducing bacteria that accelerated dynamic iron cycling by redox changes through Fe²⁺/Fe³⁺ oxidation/reduction. This self-reinforcing process amplified organo-mineral associations in reactive iron plaque, directly raising OC stabilization efficiency. Consequently, selecting rice varieties with larger ROL intensity could represent an effective strategy to increase OC stabilization in paddy soils. This study underscores the pivotal role of rice ROL for iron plaque formation and C sequestration, providing the first exploration of how ROL and aerenchyma percentage influence these processes in paddy soils.