Contribution of water dissolved organic carbon to iron (Ⅲ) reduction in paddy soils

Objective Iron Fe(Ⅲ) reduction is closely associated with the degradation of soil organic carbon and the bioavailability of nitrogen, inorganic phosphate and sulfur in submerged paddy field. This study is to investigate the contribution of water dissolved organic carbon (DOC) to Fe (Ⅲ) reduction in paddy soil, and thus to provide a better understanding on the cycles of carbon, nitrogen, phosphorus and sulfur, which are mediated by a Fe cycle coupling in paddy soils.

Methods Total 20 samples of paddy soils were collected in the typical paddy fields representation major rice production regions in China. The DOC contents in paddy soils were determined by the total organic carbon analyzer, and the DOC composition was analyzed by the fluorescence indicator and a PARAFAC analyzer of EEM spectra. Anaerobic paddy soil was incubated under anaerobic condition, and accumulation changes of Fe (Ⅲ) accompanying incubation process were measured. The Fe (Ⅲ) reduction profiles were also presented by modeling with Logistic Model. Both correlation and redundancy analyses were used to reveal the specific contribution of the humic acid-like components to microbial Fe (Ⅲ) reducing characteristics.

Results Data showed that the contents of DOC in paddy soils were ranged from 0.250 to 1.082 g/kg, and accounted for 2.06%–6.86% of the contents of soil organic carbon. The fluorescence EEM spectra of the extract solutions from paddy soils were decomposed into a model with four components by analyzing the residuals. The fluorescence intensity of the terrestrial UVC humic acid-like component and UVC+UVA humic acid-like component were represented by 0.799–4.570 and 0.830–5.273, respectively, and higher than those of aquatic visible humic acid-like component and UVA humic acid-like component. The fluorescence strength of paddy soils revealed that water dissolved organic carbon was terrestrially-derived rather than microbial-derived. There were significant differences in the Fe (Ⅲ) reduction characteristic parameters. The amorphous iron oxides in paddy soils were highly reduced after anaerobic incubation for 5 d. The humification index was positively related to the capacity of Fe(Ⅲ) reduction in paddy soils. The maximum contribution in Fe (Ⅲ) reduction capacity was confirmed to be high-molecule-weight UVC terrestrial humic acid-like component, while the contribution of the contents of water dissolved organic carbon showed a minimum response. Data analyses showed a significant relativity between the following three parameters, the Logistic kinetics parameters of microbial Fe (Ⅲ) reduction (a and V_max), the fluorescence of UVC+UVA terrestrial humic acid-like component and the UVA aquatic humic acid-like component.

Conclusion Although the DOC acts as electron donator for Fe(Ⅲ) reduction, both the humification strength of the DOC and the fluorescence intensity of each humic acid-like component play a positive role in the Fe(Ⅲ) reduction process in paddy soils.