Abstract:

Coastal wetlands are critical “blue carbon” sinks and biodiversity hotspots, but their soil organic carbon (SOC) stability is threatened by natural and anthropogenic disturbances. While these ecosystems sequester carbon rapidly, sea-level rise, land-use change, and biological invasions undermine this service. A key gap exists in understanding the mechanistic controls over the spatial partitioning of labile organic carbon (LOC) and recalcitrant organic carbon (ROC) in dynamic estuaries, where vegetation, tidal hydrodynamics, sedimentology, and salinity interact to create heterogeneous biogeochemical landscapes. Previous studies often lacked integration of detailed SOC fractionation, comprehensive environmental analysis, and advanced spatial statistics, limiting insights into the multi-scale drivers of carbon pool dynamics. This study systematically investigated SOC fractions and their controls in Chongming Dongtan Wetland (31°27′28.88″N‒31°35′12.85″N, 121°54′40.62″E‒121°55′56.06″E), a Ramsar site in the Yangtze River estuary with distinct vegetation zones (invasive Spartina alterniflora, native Phragmites australis, Scirpus mariqueter, and unvegetated mudflats) and tidal gradients. Surface soil samples (0-10 cm) were collected in August 2023 from 18 sites across three tidal elevations (high, middle, low) and four land-cover types. SOC was sequentially fractionated into water-extractable organic carbon (WEC), salt-extractable organic carbon (SEC, 0.5 mol∙L⁻¹ K₂SO₄), microbial biomass carbon (MBC, chloroform fumigation-extraction), and recalcitrant organic carbon (ROC, 6 mol∙L⁻¹ HCl hydrolysis at 115 ℃ for 16 h). Total organic carbon (TOC) was measured after carbonate removal (Shimadzu TOC-L CPN analyzer; detection limit: 0.01 mol∙L⁻¹, RSD<2%). Environmental variables included soil salinity (EC), median particle size (D₅₀), moisture, and pH. Analyses included one-way ANOVA with Tukey’s HSD, Pearson correlations, and global/local Moran’s I spatial autocorrelation (ArcGIS 10.8). Results showed high spatial variability in SOC. Mean TOC was (9.64±4.23) g∙kg⁻¹ (CV=43.9%). Vegetation was the primary biological driver: Spartina alterniflora zones had significantly higher TOC [(13.86±4.28) g∙kg⁻¹] than Phragmites australis [(8.42±2.33) g∙kg⁻¹], Scirpus mariqueter [(6.44±2.65) g∙kg⁻¹], and mudflats [(4.12±1.89) g∙kg⁻¹]. ROC dominated the SOC pool (57.55%±11.45% of TOC) and increased significantly with tidal elevation: high-tidal zones [(5.81±1.41) g∙kg⁻¹]>middle-tidal [(3.68±1.52) g∙kg⁻¹, p<0.05]>low-tidal [(2.35±1.01) g∙kg⁻¹, p<0.01]. In contrast, the labile pool averaged (1.28± 0.56) g∙kg⁻¹ (13.3% of TOC) and exhibited a north-south divide: Northern LOC [(1.89±0.42) g∙kg⁻¹] was 2-3 times higher than southern LOC [(0.67±0.21) g∙kg⁻¹], paralleled by higher MBC in the north. Spatial autocorrelation confirmed significant positive clustering for TOC (Moran’s I=0.38), LOC (I=0.42), and salinity (I=0.51), while D₅₀ showed negative autocorrelation (I= −0.35). ROC also displayed significant positive autocorrelation (I=0.29). Local Moran’s I maps identified “high-high” clusters of TOC and ROC in northern high-tidal Spartina zones (TOC 15-18 g∙kg⁻¹, ROC 6-8 g∙kg⁻¹, EC>20 mS·cm⁻¹, D₅₀>50 μm), and “low-low” clusters in southern low-tidal mudflats (TOC>4 g∙kg⁻¹, ROC>2 g∙kg⁻¹). TOC correlated positively with salinity (r²=0.68) and negatively with D₅₀ (r²=0.78); ROC correlated most strongly with tidal elevation (r²=0.62); LOC correlated positively with MBC (r²=0.83) and negatively with pH (r²=0.49). The findings are integrated into a mechanistic framework for estuarine SOC dynamics. Spartina alterniflora enhances carbon accumulation through high biomass input, sediment trapping, and potential suppression of microbial decomposition. Tidal hydrology controls carbon quality: higher elevations with prolonged anaerobic conditions promote ROC stabilization. Spatial clustering reveals predictable landscape patterns driven by vegetation-geomorphic synergy, forming carbon hotspots and coldspots. In conclusion: 1) Spartina alterniflora acts as an ecosystem engineer that increases both SOC quantity and stability compared to native vegetation; 2) tidal hydrology fundamentally regulates carbon quality, with higher elevations favoring recalcitrant carbon accumulation; 3) spatial self-organization of carbon into hotspots and coldspots is driven by vegetation-tidal geomorphology interactions. For management, northern high-tidal Spartina clusters should be prioritized as “Blue Carbon Reserves.” Restoration in low-carbon zones should favor native Phragmites australis. Maintaining natural tidal regimes and hydrologic connectivity is essential to preserve anaerobic conditions for long-term carbon stability. This framework supports the assessment and sustainable management of blue carbon in global estuarine wetlands.

Key words: Chongming Dongtan Wetland, soil, organic carbon fractions, spatial distribution patterns, salinity, median particle size

摘要：

滨海湿地生态系统具有较高的碳储存能力，而湿地土壤有机碳组分特征对于认识滨海湿地碳循环过程至关重要。该研究对崇明东滩不同植被类型覆盖的湿地开展调查与样品采集，分析了土壤有机碳组分及理化性质；通过相关性和差异性显著分析，探讨土壤有机碳组分在潮滩梯度与不同植被类型覆盖下的分布特征，以及土壤理化性质对其分布的影响。结果表明：崇明东滩湿地土壤总有机碳（TOC）质量分数均值为（9.64±4.23）g∙kg⁻¹，其中互花米草（Spartina alterniflora）湿地土壤TOC质量分数最高，为（13.9±4.28）g∙kg⁻¹，TOC与惰性有机碳（ROC）相关性最强；活性有机碳组分（LOC）含量在互花米草的聚集性明显；ROC含量表现为高潮滩区域[（5.81±1.41）g∙kg⁻¹]>中潮滩[（3.68±1.52）g∙kg⁻¹]>低潮滩[（2.35±1.01）g∙kg⁻¹]的潮滩梯度特征；崇明东滩湿地土壤有机碳组分及理化因子在南、北部呈现出不同的空间聚集性，在北部高潮滩区TOC、LOC含量较高，土壤盐度北部较高，而粒径则南部较高；湿地土壤有机碳及其组分与理化因子相关性分析显示，TOC含量与土壤含水率及中值粒径呈现显著负相关（p<0.05），而与盐度呈现显著正相关（p<0.05），崇明东滩湿地土壤的中值粒径和盐度是有机碳含量的主控因子。

关键词: 崇明东滩湿地, 土壤, 有机碳组分, 空间分布, 盐度, 中值粒径