Spectroscopic Characterization And Biodegradation Of Soil Organic Matter Under Different Vegetations Along An Elevation Gradient In The Wuyi Mountains

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. However, according to density fractionation, soil organic matter (SOM) can be separated into the light fraction (LF) and heavy fraction (HF). 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.The extent of HWEOC biodegradation was higher in EBF and DF soils with about 20% of HWEOC being mineralized, and the labile C accounted for 13-15% of the total HWEOC. HWEOC from CF soils was the least degradable with only 12% of HWEOC being mineralized. The high biodegradability of HWEOC depended on, i.e. low soil C/N ratio, low aromaticity and humification indices, small proportion of phenols and large proportion of hydrophilic carbon. During biodegradation, the UV254 absorption and the humification index (HIXsyn) of HWEOM increased significantly for most vegetation types.The concentrations of C and N in both light and heavy fractions were the highest in AM soils, while the C/N ratios of those were the greatest in CF soils across soil depths. The contribution of light fraction pool to total SOM pool was higher under CF than that under the other three vegetation types, and decreased sharply with depth in the soil profile. The LFC and LFN storages increased with increasing soil C/N ratio, litter and fine root biomass. The HFC and HFN storages increased with altitude. Soil microbial biomass C was positively correlated with the C and N storages in the two density fractions.