长江口盐沼滩面发育对有机碳深度分布的制约

Constraints for Distribution of Soil Organic Carbon with Depth due to Evolution of the Salt Marsh in the Yangtze River Estuary

通过对长江口崇明东滩高潮滩、中潮滩以及光滩柱状样的有机碳含量与碳稳定同位素组成（δ^13C）、粒度组成等的测定，研究盐沼有机碳深度分布特征与形成机制。结果表明，盐沼土壤颗粒有机碳（POC）主要赋存于粒径小于0．016mm的颗粒中，POC含量对粒径在0．002～0．004mm区间的颗粒含量变化最敏感，说明盐沼POC主要来自长江径流悬移质，这与有机碳稳定同位素结果一致。土壤POC含量与不同粒径区间颗粒含量相关关系表明，高潮滩与中潮滩柱样的泥沙级配较为接近；光滩柱样POC含量与不同粒径区间颗粒含量相关关系特征与高、中潮滩柱样的基本类似，主要不同表现在粒径大于0．016mm的粗颗粒，这很可能受控于盐沼不同高程部位动力沉积过程。盐沼植被对高、中潮滩柱样POC的贡献相当可观，个别层段高达55．6％；植被对土壤POC的贡献受到滩面过程的明显制约。滩面动力沉积过程形成盐沼垂向上独特的沙、泥纹层构造，其优良的封堵效能显著影响土壤有机碳的垂向分布。盐沼滩面动力沉积过程是塑造有机碳深度分布特征的关键因素。

Distribution of particulate organic carbon （POC） with depth was studied to elucidate the mechanism for vertical distribution of salt marsh POC, based on analyses of POC content, stable carbon isotope and grain size for samples of three cores from high marsh, middle marsh and bare fiat of Chongmingdongtan Salt Marsh in the Yangtze River Estuary. Results indicated that soil particulate organic carbon existed mainly in the fine grains with sizes less than 0.016 mm, and POC content was susceptible to changes of contents of grains in size between 0.002 mm and 0.004 mm. It was inferred that salt marsh POC originated mainly from suspended particulate matter of the Yangtze River, based on results of stable carbon isotope analysis and the correlations between POC content and contents of fine grains in different size fractions （ 〈0.016 mm） for soil samples from different cores. The correlations between soil POC contents and contents of grains in different size fractions indicated that grain size characteristics of samples from high marsh were similar to those of samples from middle marsh. Samples from bare fiat were similar in part to those from high marsh and middle marsh in the correlations between soil POC content and contents of grains in different size fractions, with main differences shown by grains in size fractions greater than 0. 016 mm. This might be due to the dynamic depositional processes at locations with different altitudes in the salt marsh. Vegetation provided considerable contributions to soil POC of core samples from high marsh and middle marsh, with the greatest value of 55.6% of soil POC contents in some depth sections, which was controlled predominantly by depositional processes on the tidal fiat. The characteristic structure of alternate muddy laminae and sandy laminae originated from dynamic depositional processes on tidal fiats, and exerted great influences on the distributions of soil POC with depth, due to the excellent protecting effects of the strata structure on soil POC. The dynamic depositional processes on tidal fiats were therefore the key factor constraining the distributions of soil POC with depth.