Effects of Agricultural Ditch Networks and Land Use on Dissolved Organic Matter Characteristics in Naoli River Basin, Sanjiang Plain, China

The Sanjiang Plain of China has experienced massive agricultural drainage since 1950s, which simultaneously created a potential the most complex network of agricultural ditches in the world. The role of agricultural ditch network structure and land use on regional dissolved organic matter (DOM) characteristics is still unclear. In current work, based on the DOM data obtained from field sampling in 2023 and the agricultural ditch network data of 2021, ultraviolet-visible (UV-Vis) absorption and the three-dimensional (3D) fluorescence excitation-emission matrix combined with parallel factor analysis (PARAFAC) were used to characterize the structural characteristics of DOM across different ditch density groups in the Naoli River Basin, a typical watershed in Sanjiang Plain, China. The results indicate that as ditch density increases, the complexity of the ditch network structure also increases. Compared with the low-density group, the content of humic acid substances in the medium and strong groups is higher (medium group: 26.33%; strong group: 27.73%), indicating that humic acid substances are more likely to accumulate in areas with high ditch density. In areas with a high proportion of natural land use (especially forests), protein-like components in water tend to be more abundant. Land use characteristics (independently explained 27%) has a greater impact on DOM than ditch network structure (independently explained 19%). Fluvic acid substances and protein-like substances are mainly controlled by land use characteristics, while humic acid substances are more dependent on ditch network structure. The interaction effect also indicated an antagonistic effect between land use characteristics and ditch network structure on DOM composition and spectral characteristics.