Biochemical Mechanisms of Cadmium Transformation and Methane Emissions in Paddy Soils Mediated by Iron Oxides with Different Crystallinities

Iron (Fe) cycling plays a crucial role in the biogeochemical cycling of both heavy metals and greenhouse gases in paddy soils. While dissimilatory iron reduction is known to drive the fate of contaminants and carbon, the specific effects of iron oxide crystallinity on cadmium (Cd) transformation and methane (CH₄) emissions in paddy soils remain poorly understood. Amorphous iron oxides, characterized by a high surface area and exceptional reactivity, contrast sharply with their crystalline counterparts, which possess greater thermodynamic stability and ordered structures. Elucidating how this fundamental mineralogical property governs both Cd transformation and methanogenic activity is critical for developing targeted strategies for soil remediation and climate change mitigation in rice-based agricultural systems.

The objective was to elucidate the underlying mechanisms by which iron oxide crystallinity governs these coupled processes.

In this study, we conducted a controlled soil culture experiment designed to simulate the typical hydrological cycles of paddy fields. The experiment incorporated alternating anaerobic and aerobic conditions, mimicking a complete flooding-draining cycle over a 40-day incubation period. Two representative iron oxides with contrasting crystallinity were selected: amorphous ferrihydrite (Fh), a poorly crystalline, high-surface-area mineral that forms rapidly under oxidizing conditions, and crystalline lepidocrocite (Lep), a more ordered and thermodynamically stable γ-FeOOH phase commonly found in seasonally reduced soils. Their impacts on soil Cd transformation and CH₄ emissions were systematically investigated using chemical extraction methods, kinetic modeling, and molecular microbiological analyses.

The results showed that the presence of iron oxides profoundly altered Cd geochemistry. Under anaerobic conditions, which are characteristic of the flooded period, both iron oxides significantly reduced the concentration of available Cd (defined as the sum of dissolved, exchangeable, and specifically adsorbed Cd) in the soil. Compared to the control treatment without iron oxide addition, the available Cd content decreased by 31.2% in the ferrihydrite treatment and by 52.7% in the lepidocrocite treatment. This reduction was primarily attributed to the transformation of available Cd into more stable fractions during the anaerobic phase. Kinetic modeling of the fractionation data provided deeper mechanistic insight, revealing that the presence of lepidocrocite promoted the transformation of available Cd into the Fe-Mn oxide bound fraction at a rate 1.76 times higher than that observed in the ferrihydrite treatment. In addition, a portion of the available Cd was redirected into other stable pools, including the organic matter-bound fraction and sulfide precipitates formed under strongly reducing conditions, ultimately contributing to a measurable increase in the residual Cd fraction, which is considered largely immobile under normal environmental conditions. During the aerobic phase of the simulated drainage period, the re-oxidation of the soil environment led to the partial remobilization of Cd in the control treatment. However, both iron oxide treatments continued to exhibit significantly lower available Cd concentrations compared to the control. Specifically, available Cd was reduced by 37.2% and 42.7% in the ferrihydrite and lepidocrocite treatments, respectively, relative to the control. Notably, the lepidocrocite treatment was more effective than ferrihydrite at inhibiting the remobilization of Fe-Mn oxide bound Cd back into available forms during this oxidative stage. This observation strongly suggests that Cd associated with, or transformed under the influence of, crystalline iron oxides is more resistant to remobilization upon aeration compared to that associated with amorphous ferrihydrite. The superior performance of lepidocrocite stems from its stronger binding energy and its structural stability under phase transformation in reducing conditions, whereas cadmium associated with ferrihydrite is more susceptible to release during the phase transformation of ferrihydrite. Overall, compared with ferrihydrite, crystalline lepidocrocite demonstrated a superior capacity for Cd immobilization throughout the entire flooding-draining alternation. This finding underscores the critical importance of mineral stability in achieving long-term, sustainable metal sequestration in dynamic soil environments. In addition to their profound influence on Cd dynamics, both iron oxides significantly suppressed methane emissions from the soil. Over the entire anaerobic incubation period, the cumulative CH₄ emissions were reduced by an average of 96.6% in the iron oxide-amended treatments compared to the control, with no statistically significant difference observed between the ferrihydrite and lepidocrocite treatments. This dramatic suppression is attributed to the role of iron oxides as competitive electron acceptors. In anaerobic soils, fermentative microorganisms break down organic matter, producing acetate and H₂, which are primary substrates for methanogenic archaea. The presence of ferrihydrite and lepidocrocite fueled dissimilatory iron-reducing bacteria, which outcompete methanogens for these common electron donors due to the higher energy yield of iron reduction. This electron competition was corroborated by molecular analysis of the soil microbiota. The decrease in the methanogen community abundance provided a mechanistic explanation for the nearly complete inhibition of methane production. Quantitative polymerase chain reaction targeting the methyl-coenzyme M reductase alpha subunit gene, which encodes a key enzyme in the methanogenic pathway, revealed that the copy numbers were significantly lower in both iron oxide treatments compared to the control. These findings collectively demonstrate that iron oxide minerals, regardless of their crystallinity, serve as a crucial link between heavy metal fate and greenhouse gas emissions in paddy soil ecosystems. By effectively adsorbing Cd and suppressing CH₄ production through competition with methanogens for electron donors, they possess the dual functionality to simultaneously mitigate Cd pollution and suppress CH₄ release. Crucially, the crystallinity of the iron oxide dictates the efficiency and stability of Cd immobilization. While amorphous ferrihydrite is effective, crystalline lepidocrocite promotes faster and more stable sequestration of Cd into non-bioavailable fractions, particularly the Fe-Mn oxide bound pool, and more effectively prevents its remobilization during oxidative events. In contrast, the suppression of methanogenesis appears to be primarily a function of the presence of bioavailable Fe(Ⅲ) as an electron acceptor, with crystallinity playing a less significant role under the high iron loading conditions of this study.

Crystalline lepidocrocite more effectively enables the simultaneous remediation of cadmium contamination and mitigation of methane emissions. The findings highlight the potential of regulating the iron redox cycle as a nature-based solution for sustainable soil management, providing important scientific evidence for simultaneously achieving Cd pollution remediation and CH₄ emission reduction in paddy soils.