Spectrofluorometric characterization of soil dissolved organic matter under different vegetation in Loess Hilly Region

【Objectives】Composition and structure of soil dissolved organic matter (DOM) are important indices of soil quality,and play significant roles in ecosystem functions by influencing carbon and nitrogen cycles. Studies of soil DOM quantities and fluorescent properties under different vegetation can provide scientific evidence for vegetation restoration on soil quality assessment. 【Methods】The traditional fluorescence spectra and three-dimensional fluorescence spectra are combined to study soil DOM. Humification indices from excitation-emission fluorescence spectra and synchronous fluorescence spectra were selected to assess source and structure of DOM, and the composition difference was researched by using 3-dimensional fluorescence spectral characteristics. 【Results】There are high contents of DOM with 0.16 mg/g for Quercus liaotungensis and Pinus tabulaeformia forestland, compared to farm and uncultivated lands with the contentss from 0.04 to 0.05 mg/g, and the DOM contents of shrub forest land such as Vitex negundo var. heterophylla and Sophora viciifolia are in the above range. The excitation-emission, synchronous, and three-dimensional fluorescence spectra show that soil DOM is mostly derived from mixing plants and microorganisms, the structures of DOM in Pinus tabulaeformia forestland is relatively simple due to low humification degree, in contrast to DOM in shrub land with higher humification degree and more complex matter structure. There are no obvious differences for protein-like matters for different vegetation and land utilization. The DOM in Quercus liaotungensis and Pinus tabulaeformia forestland has high contents for tyrosine-like protein, tryptophan-like protein, and some matters associated with biological production, but these materials are low in shrub forestland. The major differences are in humic acid-like and fulvic acid-like matters. The small molecular weight fulvic acid-like matters are predominant for soil DOM in Pinus tabulaeformia forestland, but DOM under shrub forest land has a higher level of aromatic. The ratios of fulvic acid-like and humic acid-like matters for farm and uncultivated lands are the lowest among soils with that different ground vegetation, and the DOM is with high degree of condensation. The DOM from Quercus liaotungensis and Pinus tabulaeformia forestland is difficult to condensate as a result of fulvic acid-like matters from decomposition of plant residues more easily oxidization, and higher the moisture in shady woodland. The degrees of aroma and humification of DOM from shrub, cultivated and uncultivated lands are rather high with sufficient time for condensation due to arid environment and less litter sources. 【Conclusions】The results of these studies illustrate that soil dissolved organic matter with different vegetation has different composition and structure, so fluorescence techniques can be used to study the characteristics of soil DOM as a tool to reveal composition and condensation of water soluble organic matte.