Changes in elemental composition, molecular weight and¹H NMR spectra of the water-extractable hydrophobic acid fraction in Cambisol with season and soil depth

Abstract

The chemical composition of the hydrophobic acid fraction, which is adsorbed to XAD‐8 resin at low pH and desorbed with high pH solution, of water‐extractable organic matter in soil probably influences its environmental role and, therefore, it is important to recognize the differences in its chemical composition with season and soil horizon. Hydrophobic acid fractions were collected seasonally in 2004 from Oi, Oe/Oa, A and B horizons of Cambisol and were investigated using elemental analysis, high‐performance size exclusion chromatography and 1H nuclear magnetic resonance (NMR) spectroscopy. Atomic ratios of H/C and O/C decreased with increasing soil depth in winter and summer, suggesting that the hydrophobic acid fraction from lower mineral horizons had lower saturated bond content and carbohydrate content than the fractions from upper organic horizons. Based on high‐performance size exclusion chromatography, the molecular weight at peak maximum of the hydrophobic acid fraction decreased significantly with increasing soil depth. The 1H NMR spectra of the hydrophobic acid fraction showed remarkable changes. The signal intensities of carbohydrate protons (δ 3.0–4.3 p.p.m.) in the hydrophobic acid fraction from organic horizons decreased from winter to summer and increased in autumn, while the signal of aliphatic protons (δ 0–3.0 p.p.m.) increased and decreased. In all seasons of the year, the signal intensities of carbohydrate protons were greater in organic horizons and decreased significantly with depth in mineral soil, but the signal for aliphatic protons increased. Our results demonstrate that season and soil depth influence the chemical composition of the hydrophobic acid fraction of water‐extractable organic matter.

Characterization of hydrophobic acid fractions in water-soluble organic matter in Dystric Cambisol and in a stream in a small forested watershed: Seasonal and vertical variations in chemical properties

“…A full description of the procedures of extraction and purification of the HoA fraction of water‐extractable organic matter was reported in our previous paper (Asakawa et al . 2006a).…”

“…This WEOM solution was fractionated into operationally defined HoA and non‐HoA fractions using the method of Thurman and Malcolm (1981) as modified by Asakawa et al . (2006a).…”

“…Molecular weight distribution of the stream HoA fraction was measured using HPSEC. Details of the HPSEC system, the separation conditions and the sample preparation method have been previously reported (Asakawa et al . 2006a).…”

“…In our previous work (Asakawa et al . 2006a) we found that the elemental composition, relative molecular weight and 1 H nuclear magnetic resonance (NMR) spectrum of the HoA fraction varied with season and soil depth, suggesting an effect of seasonal and spatial changes in microbial activity.…”

Properties of Fulvic Acid Extracted from Excess Sludge and Its Inhibiting Effect on β-Hexosaminidase Release

“…They consisted of peaks in the region of 7.0-8.1 ppm (aromatic H), strong peaks of 3.3-4.6 ppm (carbohydrate H), and peaks in region of 0.8-3.3 ppm (methylene and aromatic CH 2 , CH 3 ). For a more effective comparison, the proton chemical shift was quantitatively classified into four categories for protons in different chemical environments, 28,29) the results being presented in Table 3. The resonance of SS-FA showed a significantly higher percentage (19.48%) of protons on the terminal CH 3 , CH 2 and CH of methylene chains (region I) than that of ES-FA (2.98%), and the content of the proton ratio of CH 3 , CH 2 and CH protons to aromatic or carboxyl groups (region II) of SS-FA were also higher (20.48%) than that of ES-FA (13.48%).…”