典型喀斯特高基岩出露坡地表层土壤有机碳空间异质性及其储量估算方法

摘要

本研究基于详尽、系统的土壤采样调查,研究了喀斯特高基岩出露坡地典型样地(100 m×100 m)内表层土壤(0~15 cm)有机碳(SOC)含量的空间异质性特征,并以土壤斑块加和法为基准,探讨了传统空间插值方法和基于岩石出露率、土深校正的空间插值方法在喀斯特高基岩出露地区土壤表层有机碳储量估算中的适用性。结果表明,研究区SOC和容重均值分别为75.5 g·kg-1和0.8 g·cm-3,变异系数分别为30.6%与47.3%,皆呈现中等变异； SOC 半变异函数的最优拟和模型为指数模型,块金值和基台值分别为260.8与521.7,变程为52.5 m,其半变异函数分别在滞后距0~15.2 m与34.7~54.2 m范围内呈现明显的各向异性,说明在该尺度范围内微地貌与地形显著影响 SOC 的空间分布；利用土壤斑块加和法估算的样地表层 SOC 储量和碳密度分别为983.8 kg和0.1 kg·m-2,利用传统空间插值方法估算的表层SOC储量和碳密度分别为86264.0 kg和8.6 kg·m-2,利用基于岩石出露率、土深校正的空间插值方法估算的表层SOC储量和碳密度分别为2712.8 kg和0.3 kg·m-2。其中传统空间插值方法大大高估了喀斯特地区表层 SOC 储量和碳密度值,用该方法估算的 SOC 储量为该区SOC实际储量的87.7倍,其误估率为8668.4%。说明传统地统计学方法不适合估算喀斯特高基岩出露坡地表层SOC储量及碳密度。而基于岩石出露率、土深校正的空间插值方法大大降低了估算喀斯特高基岩出露坡地表层SOC储量和碳密度的误差,为该区SOC实际储量及碳密度的2.7倍。说明校正后的地统计方法在估算该区高基岩出露坡地表层SOC储量时具有一定的适用性。以上研究表明,地统计方法是表示该区SOC空间分布的有效手段,但由于传统地统计方法难以精确拟合高基岩出露坡地土壤斑块的空间分布、微地貌特征、岩石出露率以及土层深度等信息,在估算同类坡地SOC储量和碳密度时必须修正估算公式以接近实际值。%This study analyzed spatial variation features and compared assessment methods of soil organic carbon (SOC) storage in the 0-15 cm layer in high bulk-ratio slope of Karst Regions. Three SOC storage assessment methods (soil patch sum method, classical geo-statistical method and discontinuous soil interpolation method based on bulk-rock ratio and soil layer depth) were analyzed in the study. Using grid sampling method, about 107 soil patch samples were set up in a 100 m × 100 m plot. The SOC content and soil bulk density averages were respectively 75.5 g·kg-1 and 0.8 g·cm-3 with relatively moderate coefficients of variation of 30.6%, and 47.3%, respectively. The structural characteristics of SOC semivariogram followed an exponential model with nugget, sill and range values of 260.8, 521.7 and 52.5 m, respectively. Semivariograms of SOC content and soil bulk density showed strong anisotropy when the lag distance ranges were 0-15.2 m and 34.7-54.2 m, respectively, implying that micro-topography and topography significantly influenced structural variations in SOC. Based on soil patch sum assessment method, SOC storage of the plot and SOC density were respectively 983.8 kg and 0.1 kg·m-2. Also for discontinuous soil interpolation method, the estimated SOC storage of the plot and SOC density were respectively 2 712.8 kg and 0.3 kg·m-2. The estimated carbon stock of the plot and SOC density by the classical geo-statistical method were 86 264.0 kg and 8.6 kg·m-2, the carbon stock was 87.7 times of the actual value, with estimation error rate of 8 668.4%. The results suggested that the classical geo-statistical method was not suitable for estimating carbon stock and density in high bulk-ratio slope of Karst Regions. Compared with the classical geo-statistical method, revision of the bulk-stock ratio and soil depth significantly reduced estimation error of carbon stock and density by the discontinuous soil interpolation method. Therefore discontinuous soil interpolation method was more suitable for the estimation of soil carbon stock and density in such areas than the classical geo-statistical method. It was concluded that although the geo-statistical was an effective approach for analyzing spatial heterogeneity of SOC, it failed to precisely assess spatial distributions of soil patches, habitat types, bulk-rock ratios and soil thicknesses. Thus there was the need to revise the estimation methods in order to get the actual value of SOC. This study provided the scientific basis for future studies on the assessment of carbon storage in Karst Regions.