为了探讨西双版纳地区土地利用变化对土壤有机质含量及其化学组成的影响，选取了相邻的次生林、耕种6年的农田和定植3年的橡胶园样地，对其0～5 cm和5～20 cm表层土壤中有机质含量、胡敏酸的光谱学特性进行了分析。研究结果表明，次生林转变为农田之后，0～5 cm和5～20 cm 表层土壤有机质含量分别降低33.6%和23.7%；而次生林转变为橡胶园，分别降低28.6%和27.6%。胡敏酸可见-红外光谱结果显示，次生林转变为农田和橡胶园后，0～20 cm表层土壤E4/E6显著降低，这表明胡敏酸化学组成当中芳香族结构增加。傅立叶变换红外光谱结果同样表明，土地利用变化影响土壤有机质的化学组成。次生林转变为农田和橡胶园后，胡敏酸中羧基和酚基结构比例降低，而脂肪族、芳香族和多聚糖比例增加。
Previous studies have suggested that land use changes affect not only the content of soil organic matter (SOM), but also the chemical composition of SOM. The objective of this study was to confirm this finding at a site in tropical China. To understand the effect of land use changes on the quantity and quality of SOM, spectroscopic characteristics of humic acids from the topsoil (0-5 cm and 5-20 cm) and subsoil (20-40 cm) from a secondary tropical forest, a cropland, and a rubber tree plantation were investigated in Xishuangbanna, southwest China. The cropland had been cultivated for six years following the clearing of a secondary forest, and the 3-year old rubber tree plantation had been established on land that had been cultivated for 3 years following clearing of a secondary forest. Humic acids were extracted using an alkaline solution (0.1 M NaOH), and the extractions analyzed using Ultraviolet-visible spectroscopy (UV-VIS) and Fourier-Transform infrared spectroscopy (FT-IR). Six years after the conversion of secondary forest to cropland, SOM content in the 0-5 cm and 5-20 cm topsoil samples were reduced by 33.6% and 23.7%, respectively. Conversion of secondary forests to rubber tree plantations also greatly reduced SOM content by 28.6% in the top horizon (0-5 cm) and 27.6% at 5-20 cm depth. Comparison of UV-VIS spectra showed that E4/E6 ratios of humic acids from 0-5 cm and 5-20 cm depths in the secondary forest were higher than in the cropland and the rubber tree plantation soils, which indicated that the proportion of aromaticity in humic acids in the secondary forest was lower than the other two ecosystems. FT-IR spectra also indicated that land use change influenced the chemical composition of SOM. The proportion of carboxylic and phenolic groups in the humic acid was higher and the aliphatic and aromatic groups and polysaccharide-like substances were lower in the secondary forest than in the soils from the cropland and rubber tree plantation.