The coupling between iron and carbon and iron reducing bacteria control carbon sequestration in paddy soils

The coupling between iron (Fe) and carbon (C) and the activity of Fe reducing bacteria (FeRB) can strongly affect C sequestration in paddy soils. Although the geochemical cycling of C and Fe has been reported at individual sites, a lack of information exists across soil types and regions. In this study, we investigated the coupling of Fe-C and the role of FeRB across five contrasting paddy soils, which account for 60 % of China's rice cultivation area: Haplic Solonchaks (SCh), Calcaric Fluvisols in Henan (FLc1) and in Tianjin (FLc2), Cumulic Anthrosols (ATc), and Eutric Cambisols (CMe). ATc and CMe were in oxidizing environments and contained significantly higher Fe (III), free Fe oxide (Fed), amorphous Fe oxide (Feo), and organic matter complexed iron oxides (Fep) than SCh, FLc1, and FLc2 which were in reducing environments with higher relative abundance of FeRB. Nonmetric multidimensional scale analysis (NMDS) demonstrated the variability of FeRB in different soil types (r = 0.928, P = 0.001). The higher Fe-soil organic carbon (Fe-SOC) in CMe (2.27 g kg−1; 2.72 g kg−1) and ATc (1.32 g kg−1; 1.6 g kg−1) was likely due to the complexation of Fe (III) and its oxides within SOC and minimal dissimilatory Fe reduction. The existence of the "iron gate" in ATc and CMe also revealed the inhibitory effect of low quantities of Fe (II) on phenol oxidase (PO) enzyme activity and SOC mineralization. According to canonical correspondence analysis (CCA), our study indicates the importance of DOC aromaticity as characterized by UV light absorption at 254 nm (SUV254) (16.6 %), Fed (10.9 %), Feo (9.2 %) to FeRB. Moreover, the structural equation model (SEM) further revealed that SOC and DOC were regulated by PO activity, Fe, and FeRB (χ2 = 7.002, P = 0.536). Our study highlights the importance of soil characteristics on Fe-C coupling in the biogeochemical cycle through interactions among Fe morphology and microorganisms via different oxidation–reduction conditions.