Abstract:In this study, the chemical constituents of dissolved organic carbon (DOM) in upland and paddy soils under typical landforms in subtropical region were investigated in order to provide theoretical basis for evaluating its stability. The chemical constituents of DOM in upland and paddy soils from karst, hilly and plain-lake areas in southern China at fallow and ploughing periods were analyzed using an XAD-8 resin-based fractionation method and pyrolysis-gas chromatography-mass spectrometry analysis. Results showed that higher contents of hydrophilic DOC (Hi-DOC) and hydrophobic DOC (Ho-DOC) were found in paddy soils (10.2 and 33.4 mg/kg) than in upland soils (4.15 and12.8 mg/kg) at fallow period (P<0.05), whereas no significant difference of these components was found at ploughing period. As compared with fallow period, the contents of Hi-DOC and Ho-DOC in paddy soil were significantly decreased at ploughing period (P<0.05). However, those in upland soils were not greatly changed between fallow and ploughing periods. The relative proportions of organic acids in total DOM in upland and paddy soils were 31.9% and 35.6% at fallow period, but were decreased to 17.9% and 20.0% at ploughing period, respectively (P<0.05). Contrary to organic acids, the relative proportions of lipids in DOM from upland and paddy soils were 55.0% and 49.5% at fallow period, but increased up to 70.1% and 62.9% at ploughing period, respectively (P<0.05). The adonis test showed that the contents of Hi-DOC and Ho-DOC were significantly different between upland and paddy soils and between fallow and ploughing stages (P<0.05), whereas the DOM constituents were significantly different between different periods (fallow and plough) and between different landforms (karst mountain region vs hill region, hill region vs plain-lake region) (P<0.05). The random forest test showed that the organic acids and lipids involved in microbial metabolic processes, were significantly different between fallow and ploughing periods (P<0.05). However, the aromatics as a stable component of DOM was significantly different between different landforms (P<0.05). In brief, this study indicates that the DOC in paddy soil was more sensitive to periods than in upland soil, and reveals that the significant differences in microbial metabolic-related components (organic acids and lipids) and a stable component (aromatics) in soil DOM were various with different periods and landforms, which could provide a reference for DOM stability as well as SOC accumulation in upland and paddy soils.