泥炭沼泽湿地关键元素地球化学特征及其对碳排放的影响机制

Abstract

硫、铁是泥炭沼泽湿地（泥炭地）中重要的生源要素，其参与下的生物地球化学过程对泥炭地碳循环意义重大。选取德国中部两处典型的雨养型泥炭地高海拔样点（TBP）和低海拔样点（TSP），通过原位采集泥炭剖面孔隙水和可溶性气体等，研究了硫、铁元素等地球化学变化规律，结合DOC、甲烷（CH₄）和二氧化碳（CO₂）浓度分布，探讨其对泥炭地碳排放的影响。研究结果表明：（1） TBP中总还原无机硫（TRIS）浓度随深度先增后减，且上部0-87 cm平均浓度远高于87 cm深度以下，上部硫酸盐还原作用强烈。结合上部亚铁、硫化氢（H₂S）浓度分布，得知该范围内H₂S主要是通过微生物硫酸盐还原作用（BSR）生成，同时H₂S在孔隙水扩散过程中易与亚铁结合为硫化亚铁，进而生成稳定的黄铁矿，这一反应过程在约60 cm处减缓。（2） TBP、TSP两处采样点中DOC与亚铁、硫酸盐均有较强相关性，是由于地下水位的波动影响氧化还原程度以及微生物活性。两处采样点DOC均与亚铁呈显著正相关关系，表明铁氧化物在厌氧环境中被还原溶解产生亚铁，与其结合的有机碳被释放到溶液中从而导致DOC浓度的升高。TBP中DOC与硫酸盐呈显著负相关关系，表明硫酸盐作为电子受体被还原的过程中消耗酸度使pH值升高，增强了其中微生物的活性，DOC浓度由此增加。（3） CH₄与硫酸盐、TRIS浓度在剖面上均呈现相反变化趋势，表明硫酸盐输入的增加以及硫酸盐还原活动均会抑制CH₄生成。CO₂/CH₄均大于4，表明硫酸盐作为替代电子受体会使厌氧条件下碳矿化转向多CO₂和少CH₄生成。此外，亚铁对于CH₄生成一定程度上会起到低促高抑的效果，而对于CO₂的生成的影响较弱。表明硫酸盐对于CH₄和CO₂生成的影响高于亚铁。研究着重探究硫、铁等关键元素地下部生物地球化学过程对碳排放的影响机制，研究结果可为泥炭地碳排放核算提供理论支撑。;Sulfur and iron are important biogenic elements in peatland. Their participation in biogeochemical process is of great significance to carbon cycle of peatland. This study aimed to investigate impact of key elements on carbon emission of ombrotrophic peatlands, at top broad position (TBP) and top slope position (TSP) sites. The concentrations and distributions of carbon as DOC, GHG (particularly CH₄ and CO₂) were estimated through an in-situ collection of pore water and soluble gas in peat profile. Combined with the geochemical characteristics of sulfur (S) and iron (Fe) elements, their impacts on carbon emissions from peatland were discussed. The results showed that (1) the concentrations of total reduced inorganic sulfur (TRIS) in TBP first increased and then decreased with depth. The average concentration of the upper part was much higher than that of deeper part. The sulfate reduction in the upper part was strong. Combined with the concentrations and distributions of ferrous iron and hydrogen sulfide (H₂S) in the corresponding range, it is inferred that H₂S is mainly generated by bacterial sulfate reduction (BSR) in this range. Meanwhile, during the diffusion process of H₂S in pore water, it is easy to combine with ferrous iron into FeS. And then stable FeS₂ was formed. The reaction process slowed down at about 60 cm. (2) The DOC had a strong correlation with ferrous iron and sulfate in TBP and TSP because the fluctuation of groundwater level affects the redox degree and microbial activity. DOC had a significantly positive correlation with ferrous iron in the two sampling sites, indicating that iron oxide was reduced to dissolved ferrous iron in the anaerobic environment, and the organic carbon combined with ferrous oxide was released into pore water, resulting in the increase of DOC concentration. DOC had a negative correlation with sulfate in TBP, indicating that the acidity was consumed in the reduction process of sulfate as electron acceptor, and the pH value increased, which enhanced the activity of microorganism. Thus the concentration of DOC was increased. (3) The concentrations of CH₄ and sulfate, CH₄ and TRIS in the two sampling sites showed opposite trends with depth on the profile, indicating that increasing sulfate input and sulfate reduction would inhibit the formation of CH₄. The CO₂/CH₄ ratios of the two sampling sites were greater than 4, indicating that sulfate as an alternative electron acceptor may shift carbon mineralization under anaerobic conditions to more CO₂ and less CH₄ production. In addition, high concentration of ferrous iron can inhibit the formation of CH₄, and low concentration of ferrous iron can promote the formation of CH₄. But ferrous iron had a weak effect on the formation of CO₂. The results also show that sulfate concentration plays a dominative role over ferrous iron in CH₄ and CO₂ production. In this study, the effects of underground geochemical factors such as sulfur and iron on carbon emissions were also discussed, which provided more rigorous theoretical support for the correction of carbon emissions from peatlands.