Changes in elemental composition, molecular weight and 1H 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.