Spectral characteristics of dissolved organic matter in various agricultural soils throughout China

Dissolved organic matter (DOM) plays an essential role in many environmental processes, particularly in soil ecosystems. In the present study, ultraviolet-visible (UV-Vis) spectroscopy and parallel factor analysis (PARAFAC) of three-dimensional fluorescence excitation-emission matrices (3D-EEMs) were used to characterize DOM extracted from various agricultural soils across four climate regions of China. The maximum (86.01 mg L^-1) and minimum (17.39 mg L^-1) dissolved organic carbon (DOC) concentrations were found in soils from Jiangsu and Yunnan, respectively. Specific UV-Vis absorption at 254 nm (SUVA₂₅₄) for soil DOM from the temperate continental climate (TCC) region was higher than that of soil DOM in other climate regions. Three fluorescence components including UVC humic-like substances (excitation peak at 400 nm, emission peak at 525 nm), UVA humic-like substances (250(330)/430 nm), and tyrosine-like materials (220(275)/320 nm) were identified in soil DOM using PARAFAC analysis. However, there were no significant differences in the distributions of these three components for soil DOM from different climate regions. Positive correlations were found among the humification index (HIX), fluorescence index (FI), and autochthonous index (BIX). Our results demonstrate that EEMs-PARAFAC could be a feasible approach for characterizing DOM in agricultural soils from different crop systems and can be used to further study complex DOM in agricultural environments.