Characterizing the release of different composition of dissolved organic matter in soil under acid rain leaching using three-dimensional excitation–emission matrix spectroscopy

Although excitation–emission matrix spectroscopy (EEMS) has been widely used to characterize dissolved organic matter (DOM), there has no report that EEMS has been used to study the effects of acid rain on DOM and its composition in soil. In this work, we employed three-dimensional EEMS to characterize the compositions of DOM leached by simulated acid rain from red soil. The red soil was subjected to leaching of simulated acid rain of different acidity, and the leached DOM presented five main peaks in its EEMS: peak-A, related to humic acid-like (HA-like) material, at Ex/Em of 310–330/395–420 nm; peak-B, related to UV fulvic acid-like (FA-like) material, at Ex/Em of 230–280/400–435 nm; peak-C and peak-D, both related to microbial byproduct-like material, at Ex/Em of 250–280/335–355 nm and 260–280/290–320 nm, respectively; and peak-E, related to simple aromatic proteins, at Ex/Em of 210–240/290–340 nm. EEMS analysis results indicated that most DOM could be lost from red soil in the early phase of acid rain leaching. In addition to the effects of the pH of acid rain, the loss of DOM also depended on the properties of its compositions and the solubility of their complexes with aluminum. HA-like and microbial byproduct-like materials could be more easily released from red soil by acid rain at both higher pH (4.5 and 5.6) and lower pH (2.5 and 3) than that at middle pH (3.5). On the contrary, FA-like material lost in a similar manner under the action of different acid rains with pH ranging from 2.5 to 5.6.