Spectroscopic Properties of Dissolved Fulvic Acids: An Indicator for Soil Salinization in Arid and Semiarid Regions in China

This study was conducted to analyze UV-visible and fluorescence spectroscopic properties of fulvic acids (FA) from saline soils in arid and semiarid regions and to evaluate the humification degree of FA and soil salinization processes. Composite soil samples of different depths (0-20 cm, 20-40 cm, and 40-60 cm) were collected from four different halophyte communities along a saline-impact gradient around Wuliangsuhai Lake: Comm. Salicornia europaea (CSE), Comm. Suaeda glauca (CSG), Comm. Kalidium foliatum (CKF), and Comm. Sophora alopecuroides (CSA). Six humification indices (HIX) (SUVA, S275-295, A(4)/A(1), I-400/I-360, I-470/I-360, and AF(2)/AF(1)), deduced from UV-visible and fluorescence spectra of FA, were used to assess the humification degree of FA. Fulvic acids from CSA soils showed the highest average values of HIX, followed by CKF, CSG, and CSE. Fulvic acids from the CSA soils contained the most aromatic carbon structures and had the highest humification degree, followed by the CKF, the CSG, and the CSE. There were good relationships among the six HIX (P < 0.05), and I-470/I-360 was significantly correlated with the rest HIX (P < 0.01). There were larger dispersions of values within SUVA, S275-295, and A(4)/A(1) than those within I-470/I-360, I-400/I-360, and AF(2)/AF(1), indicating that the former more indistinctly differentiate humification degree than the latter. The six HIX exhibited close negative correlation with exchangeable sodium percentage (ESP) (R-2 >= 0.600 and P <= 0.003). Humification increased with the decrease of ESP. The HIX not only indicate the humification degree, but provide some insight into the processes of soil salinization. Therefore, HIX may be used as a surrogate for ESP and may be indicative of soil salinization processes. This study can provide a theoretical basis for the prevention of desertification and saline soil remediation.