黄土丘陵区退耕小流域土壤有机碳分布特征及地形植被对其的影响

针对黄土丘陵区退耕还林(草)工程实施20年固碳效果研究薄弱的问题,以典型退耕小流域为对象,在不同地形(峁坡、沟肩、沟谷)和植被类型(次生草地、撂荒山杏林、撂荒坡耕地)共布设147个样点采集0—100 cm土层样品并测定,以研究土壤有机碳(SOC)分布特征及地形、植被对其的影响。结果表明：小流域峁坡剖面(0—100 cm)土层SOC含量平均为2.43 g/kg。地形和植被类型对小流域SOC分布特征产生了重要影响：沟肩表层和剖面SOC含量均最高且显著(P<0.05)高于沟谷,但与峁坡无显著差异;次生草地表层(0—20 cm)和亚表层(20—40 cm)SOC含量均显著(P<0.05)高于撂荒山杏林和撂荒坡耕地。地统计学分析显示小流域0—20 cm土层SOC含量有最大块金值且块金系数为49.6%,即表层SOC具有最大块金效应且受到结构因素与随机因素共同影响;剖面SOC分布格局表现出与表层土壤相似的特征。总之,退耕还林(草)碳汇效应显著,且在地形和植被类型作用下呈现显著的空间异质性特征。

Distribution characteristics of soil organic carbon and the effect of topographic and vegetation on small watershed of returned farmland in the loess hilly region

The "Grain to Green" project has been implemented for more than 20 years in the loess hilly region. However, its carbon sequestration effect is rarely evaluated. To this end, a total of 147 sample sites were collected in different topography (hillslope, gully shoulder, and gully) and vegetation types (secondary grassland, abandoned apricot forest, and abandoned slope cropland) in a small watershed in the region to collect samples from 0-20 cm, 20-40 cm, 40-60 cm, 60-80 cm and 80-100 cm soil layers. The distribution characteristics of soil organic carbon (SOC) and the influence of topography and vegetation were studied by statistics and geostatistics. The results showed that the average SOC content in the 0-100 cm profile of Xifuzigou watershed was 2.43 g/kg. Topography and vegetation types had important effects on the distribution characteristics of the SOC in small watershed. The SOC content in the surface and profile of gully shoulder was the highest and significantly higher (P<0.05) than gully, but had no significant difference with hillslope. The SOC in the surface layer (0-20 cm) and subsurface layer (20-40 cm) of the secondary grassland was significantly higher (P<0.05) than that of abandoned apricot forest and abandoned slope cropland. Geostatistical analysis showed that the 0-20 cm soil layer had the largest nugget and the nugget coefficient was 49.6%, and the 0-100 cm profile''s nugget coefficient was 15.7%. The results indicated that the surface SOC had the largest nugget effect and was influenced by both structural and random factors, and the profile SOC had strongly spatial correlation which distribution was dominated by structural factors. The distribution pattern of the SOC in 0-100 cm profile was similar to that in surface soil which high values of SOC content were distributed in the secondary grassland and abandoned apricot forest, while the low values were mainly in abandoned slope cropland and dam land. At the small watershed scale, the distribution of SOC was consistent with vegetation types, but had no significant correlation with microtopography (slope, aspect and position), which indicated that the land-use/land cover dramatic changes weakened the effect of topography on SOC distribution. However, the results cannot reflect the carbon sequestration effect in the region because there are great differences in landform in the loess hilly region, and the implementation of the project varies greatly in shape and form. In conclusion, the carbon sink effect of "Grain to Green" is significant, and presents significantly spatial heterogeneity under the effect of terrain and vegetation type.