Spectroscopic characterization of hot-water extractable organic matter from soils under four different vegetation types along an elevation gradient in the Wuyi Mountains

Abstract

Water extractable organic carbon (WEOC) is the most active component in global carbon cycle and its chemical and structural characteristics most likely influence its biodegradation. Our objective was to investigate the structural characteristics of hot-water extractable organic matter (HWEOM) from soils collected at various depths (S1, S2, S3) under four different vegetation types, i.e. evergreen broad-leaved forest (EBF), coniferous forest (CF), dwarf forest (DF) and alpine meadow (AM) along an elevation gradient in the Wuyi Mountains, and to relate its structural characteristics to soil properties and biological properties. Here we examine the structural characteristics of HWEOM by UV, fluorescence and Fourier-transform infrared (FTIR) spectroscopic techniques. In the synchronous fluorescence spectra of all HWEOM samples, the main emission peaks were aromatic amino acid-like fluorophores, fulvic acid-like fluorophores and polycyclic aromatic structures with a high degree of conjugation. FTIR spectroscopy showed that HWEOM featured aliphatic C–H, aromatic C=C and carbohydrate C–O functional groups. The aromaticity, the humification indices (HIXem, HIXsyn) and the fluorescence efficiency of HWEOM from the S1 layer were significantly higher in CF than those in the other three vegetation types and, in all cases they decreased with depth in the soil profile. Soil C/N ratio was highly correlated with UV absorption, humification indices and fluorescence efficiency of HWEOM. Soil metabolic quotient (qCO 2) was highly negatively correlated with UV absorption and humification indices of HWEOM. The ratio of microbial biomass C to total organic C was significantly correlated with humification indices and fluorescence efficiency of HWEOM. Our results indicate that the HWEOM from CF soils contain more highly condensed aromatic and/or heterocyclic compounds and, at the same time, the high molecular weight and complex molecules are less mobile, leading to being preferentially retained in the upper soil layer.