Spatial variability of the molecular composition of humic acids from subtropical forest soils

Purpose Humic acid (HA) plays vital roles in controlling the conservation of soil and water in the forest soils. The structure of HA is closely related to its stability. A clear understanding of the structural difference of HA will facilitate the rational assessment and efficient regulation of forest soils. This study aimed to investigate the structural difference of HA in the forest soils, and the difference in the stability of HA was evaluated according to HA structure. Materials and methods Soils were collected from different elevations and profile depths of Jiugong Mountain (Hubei, China). Soil HA was then extracted and purified. The structure of HA was analyzed by elemental analysis, fluorescence spectroscopy, and solid-state C-13 nuclear magnetic resonance (C-13 NMR). The stability of soil HA was comprehensively accessed by structural complexity, alkyl C/O-alkyl C ratio, and hydrophobicity of HA. Results and discussion The complexity of HA structure was the greatest in soils in middle elevation (1200 m a.s.l.); it tended to decrease with soil depth in all examined soils. Alkyl C (25.1-40.1%) and O-alkyl C (21.2-44.3%) were the main structural fractions in HA samples. The percentage of alkyl C was the greatest in the HA from middle elevation while the O-alkyl C proportion was the highest in the HA from lower elevation (600 m a.s.l.). Alkyl C percentage tended to decrease with soil depth while O-alkyl C showed an opposite trend. The HA aromaticity increased with soil elevation but decreased with soil depth. The hydrophobicity and alkyl C/O-alkyl C ratio of HA was the greatest in middle elevation, and these indexes tended to decrease with soil depth. Conclusions The HA structure in the subtropical forest soils showed clear spatial variability. The structural features of HA implied that the stability of soil HA was the greatest in the middle elevation and it tended to decrease with soil depth in the examined subtropical forest.