Geochemical responses of iron and sulfur in coastal soils to sea level rise: A simulation study
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摘要: 海平面上升引起的海水浸没可显著改变沿海土壤氧化还原环境，并影响铁、硫地球化学过程。为探明渐进式海水浸没过程中土壤中铁、硫的地球化学响应，本文以活性铁和有机碳含量不同的土壤开展160 d的室内土柱培养实验，通过渐进式抬升海水水位模拟海平面上升，揭示孔隙水溶解铁（DFe）、溶解硫化物及固相活性铁、硫化物形态变化。结果表明，海水浸没后，各处理土壤孔隙水DFe浓度均明显升高，而溶解硫化物整体维持在较低水平；活性铁含量低的土壤仅在底部出现少量总还原无机硫（TRIS）积累，活性铁含量高的红壤中DFe积累更快、更高，并表现出TRIS更明显的积累；有机碳含量升高进一步促进铁异化还原和硫酸盐还原，并缩短硫酸盐还原响应的启动时间。不同深度间DFe和TRIS分布差异明显，其中，中层DFe浓度普遍高于底层和上层，各处理中TRIS均以单质硫（S0）和酸挥发性硫化物（AVS）为主，黄铁矿形成有限。铁异化还原和硫酸盐还原对海水浸没不同的响应时间、活性铁和有机碳含量共同影响土壤中铁、硫转化及其垂向分异。长期渐进式海水浸没不仅使硫化物积累垂直上移、还可能减弱活性铁对有机碳保护的“锈汇”效应，同时影响地下水和地表水化学。Abstract: Seawater immersion caused by sea-level rise can significantly alter the redox conditions of coastal soils and affect the geochemical processes of iron (Fe) and sulfur (S). To investigate the geochemical response of Fe and S in soils under progressive seawater immersion, a 160-d soil column incubation experiment was conducted using soils with different reactive Fe and organic carbon contents. Seawater table was progressively raised to simulate sea-level rise, and the changes in pore water dissolved Fe (DFe), dissolved sulfide, and solid-phase reactive Fe and sulfide species were analyzed. The results showed that after seawater immersion, DFe concentrations in pore water significantly increased in all soil treatments, while dissolved sulfide remained at relatively low levels overall. In soils with low reactive Fe content, only minor accumulation of total reduced inorganic sulfur (TRIS) occurred in the deeper layers. In contrast, reactive Fe-rich red soil exhibited faster and higher DFe accumulation, as well as more pronounced TRIS accumulation. Higher organic matter content further promoted dissimilatory iron reduction and sulfate reduction, and shortened the initiation time of sulfate reduction. Distinct vertical distributions of DFe and TRIS were observed: DFe concentrations in the middle layer were generally higher than those in the bottom and top layers. TRIS was dominated by elemental sulfur (S0) and acid-volatile sulfide (AVS) in all treatments, with limited formation of pyrite. The different response times of dissimilatory iron reduction and sulfate reduction to seawater immersion, together with the contents of reactive Fe and organic matter, jointly control the transformation and vertical differentiation of Fe and S in soils. Long-term progressive seawater inundation not only shifts the sulfide accumulation layer upward but also weakens the “rusty sink” effect of reactive Fe on organic carbon protection, while simultaneously affecting the chemistry of both groundwater and surface water.
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Key words:
- Sea-level rise /
- iron dissimilatory reduction /
- sulfate reduction /
- sulfide /
- dissolved iron /
- organic carbon
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图 1 海平面上升模拟实验装置图
Fig. 1 Schematic diagram of the experimental setup for simulating sea-level rise
图 2 孔隙水溶解铁含量（a–d）和溶解硫化物含量（e–h）随培养时间和海水上升高度的变化，虚线表示不同时间海水水位到达的高度，灰色表示溶解硫化物浓度低于检测限（1 μmol/L），×：无数据
Fig. 2 Changes in porewater dissolved iron (a–d) and sulfide (e–h) concentrations with incubation times and seawater heights. The dashed lines indicate the height reached by the seawater level at different times, and gray indicates dissolved sulfide concentrations below the detection limit (1 μmol/L), ×: no data.
图 3 培养结束后土壤柱中两种Fe(Ⅱ)形态（硫化Fe(Ⅱ) = FeAVS + Fepy；非硫化Fe(Ⅱ) = Feaca – FeAVS）（a−d）以及三种还原硫形态（AVS、S0、Spy）（e–h）净增加量
Fig. 3 Net increases in two Fe(Ⅱ) species (sulfidized Fe(Ⅱ) = FeAVS + Fepy; non-sulfidized Fe(Ⅱ) = Feaca − FeAVS) (a–d) and three reduced sulfur species (AVS, S0, and Spy) (e–h) in soil columns after incubations
图 4 培养结束后B、C、D柱底部（3.75 cm高度）和中部（25 cm高度）两个层位处总还原Fe(Ⅱ)、非硫化Fe(Ⅱ)以及TRIS含量与TOC含量的相关性
Fig. 4 Correlations of contents of total reduced Fe(Ⅱ), non-sulfidized Fe(Ⅱ), and TRIS with TOC contents at bottom (height: 3.75 cm) and middle layers (height: 25 cm) of soil columns B, C, and D after incubations
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