Abstract:

Iron oxides are widely recognized as critical regulators of soil organic carbon (SOC) persistence, since they provide reactive mineral surfaces that facilitate the formation of iron-organic carbon complexes (Fe-OC) which effectively shield organic matter from microbial decomposition and oxidative loss, thereby enabling long-term carbon storage in soils. Wetlands, as globally important ecosystems, play a disproportionately large role in carbon sequestration relative to their land area, acting both as sinks of organic carbon and as regulators of greenhouse gas emissions, and are thus highly relevant for global climate change mitigation. Despite this importance, the stability and mechanisms of SOC preservation in plateau wetlands remain insufficiently understood, particularly in terms of the contribution of Fe-OC, which has been proposed as a key pathway linking mineral protection with ecosystem carbon stability. Understanding the distribution characteristics of Fe-OC and identifying its main controlling factors in different ecosystems are therefore of fundamental importance, since such knowledge provides both theoretical insights into the wetland carbon cycling and practical guidance for wetland conservation and carbon management strategies. Against this backdrop, the present study focused on Militang wetland in northwestern Yunnan, China, a representative high-altitude plateau wetland located within Pudacuo National Park, together with adjacent grassland and forest ecosystems. This region is characterized by distinctive hydrological gradients, diverse vegetation assemblages, and contrasting biogeochemical conditions, which together create an ideal natural laboratory for investigating the regulatory mechanisms of Fe-OC across varying conditions. We systematically examined surface soils (0‒15 cm depth) from these three ecosystems, quantified Fe-OC contents, analyzed iron speciation through sequential extraction approaches, and employed statistical correlation and principal component analysis to determine the major factors influencing Fe-OC distribution and to compare ecosystem-specific patterns. The results demonstrated clear differences in Fe-OC accumulation among ecosystems. Wetland soils contained significantly higher Fe-OC [(33.3±42.6) g∙kg⁻¹] compared to forest soils [(12.8±8.7) g∙kg⁻¹, p<0.01] and grassland soils [(17.3±17.0) g∙kg⁻¹, p<0.05], underscoring the unique capacity of wetlands to promote Fe-OC accumulation. This elevated Fe-OC content can be attributed to the anaerobic or hypoxic conditions characteristic of wetlands, which slow down organic matter decomposition while facilitating the redox cycling of iron. Periodic reduction and reoxidation processes promote the formation of reactive Fe minerals that bind strongly with organic molecules, thereby enhancing carbon preservation. In addition, the high organic matter input and sustained soil moisture typical of wetland environments create favorable conditions for the formation and persistence of Fe-OC complexes. Correlation analyses further revealed significant positive relationships (p<0.05) between Fe-OC and total organic carbon (TOC), the carbon-to-iron molar ratio (OC/Fe), the proportion of Fe-OC relative to TOC (f_(Fe-OC)), and total nitrogen (TN), indicating that TOC spatial heterogeneity is the dominant factor controlling Fe-OC content. The quality and quantity of organic matter inputs not only determine the size of the available carbon pool for Fe binding but also influence microbial activity and nutrient cycling processes. The observed correlation with TN suggests that nitrogen availability, through its role in microbial metabolism and organic matter transformation, indirectly enhances Fe-OC formation. Iron speciation analysis revealed that organically bound Fe and metal-organic complexes were the dominant forms of iron across all three ecosystems, followed by Fe-Mn oxide-bound Fe, thereby highlighting the intimate relationship between iron chemistry and organic matter association. The distribution of these iron species was primarily regulated by soil moisture, OC/Fe, and TOC, factors that are closely interlinked in governing redox dynamics and carbon stabilization. In wetlands, periodic flooding and fluctuating redox conditions accelerate the dissolution and reprecipitation of Fe oxides, thereby intensifying interactions with organic matter and reinforcing Fe-OC formation. Principal component analysis revealed distinct ecosystem-specific signatures: wetland soils were strongly associated with carbon-related variables (Fe-OC, OC/Fe, TOC), suggesting that carbon input and accumulation processes are the dominant drivers of Fe-OC dynamics under saturated conditions; forest soils were characterized more by organically bound Fe mineral forms, reflecting the influence of forest litter input and relatively higher organic matter decomposition; grassland soils exhibited an intermediate pattern, where both carbon indicators and iron speciation jointly influenced Fe-OC dynamics, consistent with their transitional position in hydrological and vegetation gradients. Collectively, these results emphasize the crucial role of soil moisture-driven heterogeneity in TOC, along with nutrient status and iron speciation, in shaping Fe-OC characteristics across plateau ecosystems. The elevated Fe-OC levels observed in wetlands reflect not only favorable hydrological and redox conditions but also the synergistic interaction between abundant organic inputs and reactive iron phases. These findings provide novel evidence that wetlands, compared with adjacent grasslands and forests, function as more effective reservoirs for Fe-OC and possibly contribute disproportionately to SOC sequestration. From a management perspective, this suggests that conservation or restoration strategies aimed at maintaining stable water regimes and promoting anaerobic soil conditions are likely to enhance Fe-OC accumulation and improve long-term carbon sequestration in plateau wetlands. Integrating such strategies with vegetation management and microbial process regulation could further strengthen the resilience of these ecosystems to environmental change. It should be noted, however, that this study primarily focused on the quantification of Fe-OC content and associated controlling factors, while the direct mechanistic link between Fe-OC dynamics and carbon fluxes such as CO₂ and CH₄ emissions remains unquantified. Although the strong correlations with TOC indicate a significant role of Fe-OC in shaping soil carbon pools, the extent to which Fe-OC acts as a stabilizer or amplifier of greenhouse gas emissions under changing environmental conditions requires further investigation. Future research incorporating microscopic characterization techniques such as spectroscopy, isotope tracing approaches, and long-term in situ monitoring of carbon fluxes will be essential to unravel the pathways and stability of Fe-OC and to validate these findings across broader spatial and temporal scales. Such integrative studies would not only deepen our understanding of the wetland carbon cycle but also provide a stronger scientific foundation for modeling the role of plateau wetlands in global carbon budgets and for developing evidence-based strategies for wetland management, conservation, and climate mitigation.

Key words: iron-bound organic carbon, plateau wetlands, soil organic carbon, carbon sequestration, controlling factors

摘要：

铁氧化物介导的矿物保护是土壤有机碳长期固存的核心机制。为探究湿地土壤铁结合态有机碳（Fe-OC）含量特征，铁形态及其调控因素，该研究以滇西北弥里塘高原湿地及周边草地、森林为对象，对比分析不同生态系统表层土壤中Fe-OC的含量特征、铁赋存形态及其调控因素。结果表明：1）湿地Fe-OC质量分数[（33.3±42.6）]g∙kg⁻¹]显著高于森林[（12.8±8.7）g∙kg⁻¹，p<0.01]和草地[（17.3±17.0）g∙kg⁻¹，p<0.05]；2）Fe-OC与总有机碳（TOC）、碳铁摩尔比（OC/Fe）、Fe-OC占TOC比例（f_(Fe-OC)）、全氮（TN）均呈显著正相关（p<0.05），表明土壤TOC的空间异质性是影响Fe-OC含量特征的主要因素；3）湿地、草地与森林土壤中的铁均以有机结合态和金属有机物络合态为主，其次为铁-锰氧化物结合态，土壤水分质量分数、OC/Fe和TOC是调控土壤中铁形态的主要因素；4）主成分分析结果显示，湿地与森林之间存在较为明显的差异，其中湿地土壤特征主要由碳指标（Fe-OC，OC/Fe，TOC）驱动，而森林土壤则由有机结合态铁矿物驱动，草地土壤作为二者间的过渡区域则受到碳指标和铁矿物形态的共同影响。该研究揭示了由土壤水分质量分数差异驱动的TOC空间异质性、营养盐和铁形态等关键Fe-OC含量特征驱动因素，为高原湿地碳管理提供科学依据。

关键词: 铁结合态有机碳, 高原湿地, 土壤有机碳, 碳固存, 调控因素