Photochemical enrichment of dissolved organic matter from different soils of a 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 environment 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 soils from lower basin of Dagu River (DGR) in the laboratory to study the influence of salinity and soils 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 dark cultures. PCA and Mantel 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 soils with high organic carbon content and aged soils with low δ¹³C_SOM released more DOM and humic-like components. Although DGR is impacted by intruded seawater, high correlation coefficient between the content of photoreleased DOM and total DOM content in seawater cultures still pointed out the important contribution of soil photodissolution to DOM reservoir of tidal rivers. Considering high proportion of humic-like components in photoreleased DOM, photochemical transformations of soils in tidal rivers would promote carbon export flux of estuaries to open seas. This study emphasizes the importance of soil photodissolution of tidal rivers in carbon transfer from lands to oceans.