A new study has found that the reducing capacity (RC) of some aquatic sediments fluctuates much more sharply than that of others under alternating oxic and anoxic conditions, with differences in reactive iron availability and iron cycling helping explain their contrasting responses.

The findings shed new light on the factors controlling sediment reducing capacity and could help inform more targeted management of aquatic sediments.

The study, published in Environmental Science & Technology, was conducted by a research group led by Prof. LI Qingman from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences (CAS).

Sediment RC is a key indicator of benthic habitat quality. Its stability influences the biogeochemical cycling of organic carbon, nutrients, and pollutants in aquatic systems. RC consists mainly of reducing organic substances (RedOr), ferrous iron [Fe(II)], and reduced sulfur species (Sⁿ, −2 ≤ n < 0).

In natural waters, wind-driven waves, bioturbation, and water-level fluctuations continually expose the sediment-water interface to alternating oxic and anoxic conditions. However, how these changes affect sediment RC and its components, and what controls their stability, remain unclear.

To investigate these questions, the researchers collected surface sediments from Honghu Lake (HH), Xiashan Reservoir (XR), Yudong Reservoir (YR), and Zhanghe Reservoir (ZR). They conducted controlled incubation experiments under alternating oxic and anoxic conditions, tracking changes in sediment RC and its major reducing components. They also analyzed iron speciation to investigate the mechanisms governing RC stability in different sediment types.

Based on the compositive reducing capacity index, sediments from Honghu Lake and Xiashan Reservoir were classified as RedOr-Sⁿ type, while those from Yudong Reservoir and Zhanghe Reservoir were classified as RedOr-Fe(II) type. During the 54-day experiment, the sediments underwent four stages: initial anoxia, oxidation, renewed anoxia, and reoxidation.

RC in both sediment types followed the same general pattern: it decreased during oxidation, recovered under anoxia, and declined again upon reoxidation. However, the magnitude of change was much greater in RedOr-Sⁿ sediments.

During the anoxic recovery stage, RC in RedOr-Sⁿ sediments increased by 97.5 ± 61.5 cmol e^- kg^-1 dry weight (DW) on average, 4.2 times the increase observed in RedOr-Fe(II) sediments. During reoxidation, RC in RedOr-Sⁿ sediments decreased by 102.0 ± 61.1 cmol e^- kg^-1 DW, approximately three times the decline observed in RedOr-Fe(II) sediments. These results indicate that RedOr-Sⁿ sediments are more sensitive to redox alternation.

The contributions of individual reducing components also differed between sediment types. RedOr was the dominant driver of RC variation in RedOr-Sⁿ sediments, contributing 55.8%, 79.0%, and 78.7% during oxidation, anoxic recovery, and reoxidation, respectively.

In RedOr-Fe(II) sediments, Fe(II) made a much greater contribution to RC dynamics, while reduced sulfur species contributed relatively little. These results indicate that RedOr broadly governs RC dynamics, while the relative roles of Fe(II) and Sⁿ depend on sediment reductive type.

The researchers then examined how the abundance and reactivity of different reducing components affected RC stability. Under anoxic conditions, RedOr-Sⁿ sediments showed simultaneous increases in the abundance and reactivity of RedOr, Fe(II), and Sⁿ. By contrast, RedOr-Fe(II) sediments mainly showed enhanced Fe(II) reactivity, with limited changes in RedOr and Sⁿ.

Iron Cycling Mechanisms

Further analyses revealed that differences in RC stability between the two sediment types were associated with distinct iron-cycling mechanisms.

RedOr-Sⁿ sediments contained relatively low levels of easily reducible and reducible Fe(III) fractions, while magnetite-associated iron was more prominent. Under anoxic conditions, the limited reactive Fe(III) pool was rapidly consumed. Magnetite and sulfate subsequently became more important electron acceptors, promoting the accumulation of reducing substances and increasing sediment RC.

In contrast, RedOr-Fe(II) sediments contained abundant easily reducible and reducible Fe(III), which together accounted for an average of 65.7% of total iron. These reactive Fe(III) phases could accept electrons released from electron donors under anoxia, thereby limiting the accumulation of reducing substances and maintaining relatively stable RC.

Based on these mechanisms, the researchers proposed differentiated management strategies for different sediment types. For RedOr-Sⁿ sediments, management should prioritize limiting RedOr accumulation and sulfide formation. For RedOr-Fe(II) sediments, efforts should focus on maintaining reversible iron cycling and stabilizing the pool of reactive Fe(III).

According to the researchers, the findings also demonstrate how controlled redox perturbations can help distinguish sediment types and identify the key components governing their reducing capacity, providing a scientific basis for sediment risk assessment and targeted management.

Conceptual model of RC dynamics, component changes, and iron cycling in RedOr-Sⁿ and RedOr-Fe(II) sediments. (Image by IHB)