Exogenous Organic Carbon Strongly Affected the Coupling of Nitrate-dependent Fe(II) Oxidation in Coastal Saline-alkaline Paddy Soil

The redox reaction between nitrate (NO3-) and Fe(II) is important to the denitrification loss of nitrogen (N) in paddy soil, while the role of organic carbon in this process is still uncertain. The effects of exogenous organic carbon in nitrate-dependent Fe(II) oxidation (NDFO) were studied with lactate and acetate addition. Six treatments were established: (i) S1 (lower organic carbon) and S2 (higher organic carbon); (ii) S1 + lactate (S1-L) and S2 + lactate (S2-L), which were added with 10 mM lactate; and (iii) S1 + acetate (S1-A) and S2 + acetate (S2-A), which were added with 10 mM acetate. The incubation experiment was conducted for 12 days, and time-series samples were collected. Lactate could compete with Fe(II) and inhibited Fe(II) oxidation. In both S1 and S2 soils, the lactate inhibited Fe(II) oxidation, but acetate promoted Fe(II) oxidation. In S1 soil, exogenous organic carbon could accelerate the reduction rate of NO3- and promote the reduction of NO3- to produce NH4+. Compared with S1 treatment, the NO3- reduction rate in S1-L and S1-A treatments was increased by 11.8% and 30.8%, respectively. While in the group of S2 treatment, the addition of exogenous organic carbon showed no significant effect on the reduction of NO3-. Lactate could reduce N loss by competing with Fe(II) as an electron donor and facilitating the conversion of NO3- to NH4+, but acetate could promote the oxidation of Fe(II) as a carbon source. This study highlighted the importance of organic carbon in predicting Fe and NO3- transformation in coastal saline-alkaline paddy soil.