Photochemical enrichment of dissolved organic matter from different soils of tidal river basin: Significance to estuarine carbon cycle

Eroded soils sustain a substantial part of organic matter in tidal rivers adjacent to estuaries, and photochemical transformations of soils in tidal rivers would influence estuarine elemental cycles. However, complex aquatic environments and diverse soil sources complicate the enrichment of dissolved organic matter (DOM) photoreleased from soils. Here, we conducted a seven-day irradiation experiment for seven kinds of soil from the lower basin of Dagu River (DGR) in the laboratory to study the influence of salinity and soil properties on DOM chemistry by characterizing the content and optical properties of DOM. Results showed that all soils of light cultures released higher amount of DOM and humic-like components than those of dark cultures. PCA and Mantel’s analysis found that salinity and soil properties significantly influence the production of photoreleased DOM, especially humic-like components. Salinity could inhibit the photodissolution of soils and, and aged soils with low δ13CSOM released more DOM and humic-like components. Although DGR is impacted by intruded seawater, high correlation coefficient between the content of photoreleased DOM content in seawater cultures still pointed out the important contribution of soil photodissolution to the DOM reservoir of tidal rivers. Considering high proportion of humic-like components in photoreleased DOM, photochemical transformations of soils in tidal rivers would promote the export flux of carbon from estuaries to open seas. This study emphasizes the importance of soil photodissolution of tidal rivers in carbon transfer from lands to oceans.

毗邻河口的潮汐河流中，侵蚀土壤维系着相当比例的有机质，而潮汐河流中土壤的光化学转化过程会影响河口的元素循环。然而，复杂的水生环境与多样的土壤来源，使得从土壤中光解释放的溶解性有机质（Dissolved Organic Matter, DOM）的富集过程变得复杂化。本研究于实验室中针对大沽河下游流域（Dagu River, DGR）的7种土壤开展了为期7天的辐照实验，通过表征DOM的含量与光学特性，探究盐度与土壤性质对DOM化学特征的影响。实验结果显示，所有光照培养组土壤释放的DOM与类腐殖质组分含量均显著高于暗培养组。主成分分析（Principal Component Analysis, PCA）与曼特尔检验（Mantel's test）结果表明，盐度与土壤性质均会显著影响光解释放DOM的生成过程，尤其是类腐殖质组分。盐度会抑制土壤的光溶解过程，而δ¹³C_SOM较低的老化土壤会释放更多DOM与类腐殖质组分。尽管大沽河下游流域受到海水入侵的影响，但海水培养组中光解释放DOM的含量仍呈现较高的相关系数，这表明土壤光溶解过程对潮汐河流DOM库具有重要贡献。鉴于光解释放DOM中类腐殖质组分占比颇高，潮汐河流中土壤的光化学转化过程会推动碳从河口向外海的输出通量。本研究强调了潮汐河流土壤光溶解过程在陆地-海洋碳转移过程中的重要性。