Variation in the Content and Fluorescence Composition of Dissolved Organic Matter in Chinese Different-Term Rice–Crayfish Integrated Systems

This study examines the fluorescence characteristics of dissolved organic matter (DOM) in soils from different periods of rice–crayfish integrated systems (RCISs) in China. Utilizing three-dimensional excitation–emission matrix (3D-EEM) fluorescence spectroscopy, the study investigated the hydrophobicity, molecular weight distributions, and fluorescence properties of DOM in 2-, 5-, and 7-year RCIS operations, with rice monoculture (RM) serving as a control. The findings indicate that in the initial 2 years of an RCIS, factors such as rice straw deposition, root exudates, and crayfish excretions increase dissolved organic carbon (DOC) release and alter DOM composition, increasing the humic acid content in the soil. As the system matures at 5 years, improvements in soil structure and microbial activity lead to the breakdown of high-molecular-weight humic substances and a rise in small-molecular-weight amino acids. By the 7-year mark, as the aquatic ecosystem stabilizes, there is an increase in humic substances and the humification index in the soil DOM. These variations in DOM properties are essential for understanding the effects of integrated farming systems on soil quality and sustainability.