Humic substances are extremely important attributes of soil quality but the impacts of long-term fertilization on their composition and structure remain controversial and are still not fully understood. A study, employing various methods was conducted to elucidate the influence of long-term fertilization practices on the composition and structure of humic substances. Soil samples were collected from eight treatments at the Red Soil Experimental Station of the Chinese Academy of Agricultural Sciences. Data were subjected to analysis of variance (ANOVA), using the MSTAT-C software package. In general, the Fourier transform-infrared (FT-IR) spectra discriminated bands between humic fractions. The humic acid (HA) from the NPK plus manure (NPKM) and manure alone (M) treatments showed higher values of the O/R ratios (1.24 and 1.18, respectively) as compared to that from the 1.5 NPKM and fallow (H) treatments, which decreased these ratios (0.66 and 0.85, respectively). Except for NPKM plus wheat-soybean-sweet potato between the lines (NPKMR), the O/R ratios of the fulvic acid (FA) did not differ among the treatments. However, the humin (HM) from H and the control (CK) displayed the highest value of O/R ratios (about 3.23 for both). Humic substances from organic treatments showed more aliphatic nature, whereas more condensed-alkali soluble humic substances were formed in the unfertilized and mineral treatments. Among fractions, the HA formed more phenolic or aromatic structures than did the FA or HM. In addition, the C, H, N and S contents of humic substances increased significantly, whereas their oxygen content decreased.
Key words: Long-term fertilization, soil quality, humic substances, Fourier transform-infrared (FT-IR) spectroscopy, elemental composition.
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