Iron-bound organic carbon is conserved in the rhizosphere soil of freshwater wetlands

The greater number and activity of microorganisms in the rhizosphere than bulk soils is expected to affect iron (Fe) and carbon (C) biogeochemical cycling. We investigated the coupled relationships among Fe, C, and Fe-reducing bacteria (FeRB) in the rhizosphere and bulk soils of Calamagrostis angustifolia and Carex lasiocarpa in a freshwater wetland of international importance (i.e. Ramsar site) in Northeast China. Nonmetric multi-dimensional scaling analysis showed distinct clusters of FeRB in the ordination space of C. angustifolia and C. lasiocarpa associated with the rhizosphere (R = 0.707, p = 0.002 and R = 0.830, p = 0.004, respectively). The relative abundance of FeRB was significantly (p < 0.05) greater in the rhizosphere (3.3%) than the bulk soil (2.6%). The smaller Fe-bound organic carbon (OC-Fe) concentration in bulk soil could be the result of dissimilatory Fe reduction by FeRB (e.g. Anaeromyxobacter, Geobacter, Clostridium, and Bacillus). On average, there was significantly (p < 0.01) more OC-Fe in the rhizosphere soil of C. angustifolia and C. lasiocarpa (7.86 g OC-Fe/kg) than in bulk soils (2.36 g OC-Fe/kg). Structural equation modelling showed that FeRB and Fe oxides explained 65% of the variance in the OC-Fe concentration. Furthermore, the Fe(III) concentration (r = 0.62, p < 0.001) and organically complexed Fe oxides (r = 0.63, p < 0.001) significantly and directly affected the OC-Fe concentration, for aromatic, phenolic and carboxyl compounds bind preferentially to Fe oxides based on FTIR analysis. We conclude that the coupled Fe-C relationships will lead to an accumulation of OC-Fe in the rhizosphere soil, relative to bulk soil, in these freshwater wetlands.