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Concentrations and migration features of dissolved organic carbon in the soils of slope lands in Karst area

LIU Tao-ze1,2, LIU Cong-qiang1, ZHANG Wei1, TU Cheng-long1(1.State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;2.Graduate School of the Chinese Academy of Sciences, Beijing 100049, China)  
In order to understand the mechanism of carbon cycle in karst areas, surface soils and soil profiles were collected at the different topographic locations of two typical slope lands in a Karst area near Dacai township, Guangxi Province.Clay, pH, soil organic carbon(SOC), dissolved organic carbon(DOC), and stable carbon isotope(δ13C) were analysed.Fresh leaves and litters of dominant plants were also sampled for δ13C determination.Higher SOC and DOC concentrations in surface soils were measured in the slope land with shrub vegetation(113g/kg and 188mg/kg) than with grass vegetation(55.6g/kg and 173mg/kg), however, increased DOC, decreased SOC was observed in surface soils down the slope lands.δ13CDOC of the surface soils with shrub vegetation, varying in-15.1‰~-22.1‰, were close to their δ13CSOC(-14.3‰~-21.9‰) while δ13CDOC with grass vegetation varied in a narrow range of-20.0‰~-21.0‰.However, δ13CDOC and δ13CSOC of surface soils were apparently different from those of plant leaves(-11.3‰~-13.9‰) and litters(-26.7‰~-28.9‰).These results proved that soil DOC in this karst area was mainly originated from SOC.DOC in soil profiles from different topographic locations varied similarly, with high DOC in top 30cm, and decreasing sharply with depth.However, variation of δ13CDOC in the soil profiles was complicated, and peak δ13CDOC was observed at 20~30cm depth in the profile with shrub vegetation while at 20cm depth with grass vegetation.Decomposition degree of SOC was supposed to be responsible for variation of δ13CDOC with depth in soil profiles, which could be a good indicator for DOC migration in soil profiles.
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