Spectral Characteristics of Dissolved Organic Matter (DOM) in Leachate Released From Agricultural Soil Irrigated With Reclaimed Water

Agricultural irrigation is effective for the reutilization of reclaimed water, and the natural advantages and operational feasibility have been generally approved. Agricultural irrigation with reclaimed water can achieve acceptable ecological, social and economic benefits, while organic matters in the soil might lose or change during the irrigation process, and the related issue has received insufficient attention. Dissolved organic matter (DOM) was selected as the analysis target, and the scientific issue was to reveal the variation mechanism of DOM spectral information in leachate released from the irrigated agricultural soil. Samples of reclaimed water (from wastewater treatment plant) and surface water (from the river, for contrast) were used, and a one-dimensional undisturbed soil column was applied to study the irrigation effect on soil and soil leachate. The electronic scanning microscopy (SEM), ultraviolet-visible spectroscopy (UV-Vis) and three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEMs) were involved in reveal the fundamental information of DOM released from soil irrigated with water samples. The results show: that soil aggregates appear lose structure and rough surface after irrigation, and small soil particles gather and stick together. The contents of organic matter decrease with the increased soil depth, and DOM content reaches maximal at 10 similar to 20 cm of the soil layer. The concentrations of organic matter (0 similar to 30 cm of soil layer) and DOM (0 similar to 20 cm of soil layer) increase after irrigation, suggesting the interception effect of water organic compounds by soil samples. The UV-Vis spectra of leachate are similar, and the unsaturated double-bond conjugate structures cause the absorption regions at 240 similar to 270 nm in DOM. The lower aromatization degree and more straightforward structure of DOM can be detected in soil leachate irrigated with reclaimed water, and the content of fulvic acid is higher than that of humic acid. The only fluorescence peak of DOM appears in the E-x/E-m = 260 similar to 270/420 similar to 430 nm, regarded as a humus-like component. Compared to the previous results, it is deduced that amino acid-like and protein-like components might be adsorbed and retained in soil. Bio-source (endogenous) DOM plays a more important role than exogenous DOM in soil leachate. The achievements are significant to estimate further the circulation, transportation and ecological effect of DOM or organic matters in the reclaimed water-irrigated agricultural soil systems.