喀斯特地区坡地土壤可溶性有机碳的分布特征

以广西喀斯特地区2个不同退化程度的典型坡地作为研究对象,测定了坡地表层和不同地形部位(坡肩、坡背、坡腰和坡脚)剖面土壤可溶性有机碳(DOC)含量和δ13CDOC值,分析了土壤中DOC的迁移变化规律及影响因素.结果表明,DOC含量主要受到有机质输入?地形和土壤质量等因素影响.灌丛坡地表层土壤中DOC含量要高于草丛坡地,2个坡地表层土壤中DOC含量均随坡面向下而升高,与土壤有机碳(SOC)变化呈相反趋势.而在坡地各个地形剖面中,除灌丛坡地的坡肩和坡背2个剖面土壤中DOC含量随土层深度表现出一定的升高趋势,其他剖面中DOC含量均随土层深度增加而降低.灌丛坡地表层土壤中δ13CDOC值变化幅度明显高于草丛坡地,范围在-15.1‰~-22.1‰,与坡地植被和δ13CSOC值变化密切相关;而草丛坡地表层土壤δ13CDOC值范围在-19.98‰~-20.96‰,其变化幅度很小,与土壤δ13CSOC值变化相似,但与地表植被变化关系不明显,表明了DOC来源主要受到SOC的影响;而各个地形剖面中δ13CDOC值变化较为复杂,但灌丛坡地各剖面变化幅度明显高于草丛坡地相应剖面.剖面中有机质分解程度和输入量决定了δ13CDOC值变幅大小,因此土壤剖面中δ13CDOC值随深度变化的趋势,能够很好地反映剖面土壤中DOC迁移转化过程.

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

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.