Spatial Distribution and Fluorescence Properties of Soil Dissolved Organic Carbon Across a Riparian Buffer Wetland in Chongming Island, China

The migration of soil dissolved organic carbon (DOC) from terrestrial to aquatic environments has important impacts on the adjacent water quality and the transport of organic and inorganic contaminants. However, few studies have investigated the sources and properties of DOC in riparian zones. A total of 34 soil samples were collected across four riparian buffer zones (Zones A–D) on Chongming Island, China. The vertical distributions of soil organic carbon (SOC) and DOC, fluorescence excitation-emission matrix (EEM) spectra of DOC and the optical indices, including fluorescence index (FI), index of recent autochthonous contribution (BIX), and humification index (HIX), were measured across the riparian environment to investigate the sources and fluorescence properties of DOC. The results showed that SOC stored in the surface soil (0–30 cm) accounted for 40% of the total soil profile SOC. The DOC accumulated in Zones A–C, which accounted for 5% of the SOC. The fluorescence EEM spectra of DOC showed that DOC contained humic-like and protein-like substances, which were mainly derived from recent plant debris by microbes. A large amount of humic-like substances were sorbed to minerals in the surface soil (0–30 cm). In addition, the riparian topography and soil physico-chemical properties (pH, EC and moisture) dictated the transformation and transport of DOC. The results suggested that EEMs could reveal the source of DOC in riparian soil systems, and that optical indices were complementary tools that revealed the characteristics of soil DOC and provided supplemental evidence about DOC sources.