Abstract
The binding characteristics of organic ligands and minerals in fulvic acids (FAs) with Al are essential for understanding soil C sequestration, remain poorly understood. In this study, Fourier transform infrared (FTIR) spectroscopy combined with two-dimensional correlation spectroscopy (2DCOS) analysis was applied for the first time to explore the binding of Al with organic ligands and minerals in soil FAs. For these analyses, two contrasting treatments were selected from a long-term (i.e., 22-year) fertilization experiment: chemical (NPK) fertilization and swine manure (SM) fertilization. The results showed that the long-term application of organic and inorganic fertilizers to soils had little effect on the compositions of the fluorescent substances and organic ligands in the soil FAs. However, long-term SM fertilization increased the weathered Al and Si concentrations in the soil FAs compared with long-term chemical fertilization. Furthermore, organic ligands in the soil FAs were mainly bound with Al in the NPK treatment, whereas both organic ligands and minerals (Al-O-Si, Si-O) were bound with Al under the M fertilization conditions. Both transmission electron microscopy (TEM) images and X-ray diffraction spectra demonstrated that amorphous and short-range-ordered nanominerals were abundant in the soil FAs from the SM plot in contrast to the soil FAs from the NPK plot. This result illustrates the role nanominerals play in the preservation of soil FAs by during long-term organic fertilization. In summary, the combination of FTIR and 2D correlation spectroscopy is a promising approach for the characterization of the binding capability between soil FAs and Al, and a better understanding FA-Al binding capability will greatly contribute to global C cycling.
Highlights
Humic substances (HSs), the dominant component of soil organic matter (SOM), are formed by the decomposition of plant, animal, and microbial materials
As one of the main components of SOC, the soil fulvic acids (FAs) in the swine manure (SM) plot after 22 years of fertilization treatment contained a higher DOC concentration and Electrical conductivity (EC) than the FAs in the NPK plot (p, 0.01, Table 1), which was consistent with the results that showed that the SOC concentrations in the SM plot were significantly (p,0.01) greater (,5465%) than those in the NPK plot (Table 1)
Fluorescence excitation-emission matrix (EEM) spectra demonstrated that three fluorescence peaks, i.e., peaks A, B, and C, which were located at Ex/Em of 230/420, 250/420, and 330/420, respectively, were present in the soil FAs from both the NPK and SM plots (Figures 1-a and 1b) and were attributed to fulvic-like and humic-like substances, respectively [8,18,28]
Summary
Humic substances (HSs), the dominant component of soil organic matter (SOM), are formed by the decomposition of plant, animal, and microbial materials. They are complex, heterogeneous, organic compounds that possess different structures and solubilities and a wide range of molecular weights [1,2,3]. HSs are categorized into the following three groups on the basis of their solubility: humic acids (HAs), fulvic acids (FAs), and humins. Among these groups, FAs have the lowest -molecular -weight, with molecular weights ranging from approximately 500 to 2000 Da, and they contain the highest oxygen content in their complex humic groups [4]. Studies have demonstrated that nanominerals are abundant in soil FAs, which affect the nanominerals’ binding behavior with metals [7,8]
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