UV-visible spectroscopic properties of dissolved fulvic acids extracted from salined fluvo-aquic soils in the Hetao Irrigation District, China

The elemental composition and the UV-visible spectroscopic properties of dissolved fulvic acids (FA) extracted from salined fluvo-aquic soils in the Hetao Irrigation District were examined to evaluate the humification degree of FA and salinisation processes of soils. Composite soil samples of different depths (0-20, 20-40, 40-60, 60-80 cm) were collected from four native halophyte soils [communities Salicornia europaea (CSE), Suaeda glauca (CSG), Kalidium foliatum (CKF), Sophora alopecuroides (CSA)] and two furrow-irrigated fields [corn (CFD), wheat (WFD)] along a saline impact gradient. Seven humification indices (HIXs: C/N, SUVA, E(2)/E(3), E(2)/E(4), S(275-295), S(350-400), and S(250-400)), deduced from elemental analysis and UV-visible spectroscopy of FAs, were used to evaluate the humification degree. The humification degree of FA from the WFD soil was the highest, followed by CFD, CSA, CKF, CSG, and CSE. There were significant relationships between the seven HIXs (P < 0.05). The exchangeable sodium percentage (ESP) exhibited close correlations with the seven HIXs, and the humification degree increased with decreasing ESP. The HIX can not only indicate the humification degree of soil organic matter but also characterise the salinisation processes of soils.