南方红壤丘陵区侵蚀沟道内土壤团聚体及有机碳特征

为了更好的揭示特殊地形下水蚀过程对土壤结构和有机碳含量分配的影响，选取典型南方红壤丘陵区-青原山小流域为研究区，采用核素137Cs示踪技术研究小流域侵蚀沟道的水土流失现状，分析了沟道侵蚀对土壤团聚体稳定性及有机碳含量的影响。结果表明：侵蚀沟道的坡顶处137Cs含量最高，且高于背景值，属于沉积区，而坡上、坡脚属于中度侵蚀，坡中属于轻度侵蚀；侵蚀沟道顺坡而下侵蚀过程依次表现为绝对沉积、绝对侵蚀、相对沉积和绝对侵蚀，其中植被和地形因子是主导因素；沉积区相比于侵蚀区平均质量直径（Mean Weight Diameter，MWD）和大团聚体含量（粒径≥0.25 mm）更高，侵蚀区中相对沉积的坡中有着更稳定的土壤团粒结构；沉积区各个粒径的土壤团聚体有机碳含量均高于侵蚀区，侵蚀区的土壤团聚体有机碳更趋向于均匀分配，土壤理化性质的空间差异也会影响土壤团聚体有机碳含量。侵蚀沟道中土壤侵蚀模式与传统坡面并不一致，土壤结构及相关碳组分主要受地形和植被支配下的土壤侵蚀程度影响。关键词：土壤；侵蚀；侵蚀沟道；团聚体；有机碳；137Cs

Characteristics of soil aggregates and organic carbon in eroded gully in red soil region of Southern China

This study aims to present the influence of water erosion on the soil structure and distribution of organic carbon in a special terrain. The samples were collected in a small watershed of Qingyuan Mountain, a typical hilly area of red soil in southern China. The nuclide 137Cs tracer technology was used to analyze the water and soil loss during the gully erosion in the small watershed. An attempt was also made on the influence of soil erosion on the stability of soil aggregates and the content of organic carbon in the gully erosion area. Three soil depths (0-10, >10-20, >20-30 cm) were set to identify soil aggregates, and the scouring features of soil aggregates and organic carbon. The whole soil samples were gathered at the depth of 0~20 cm by the soil auger in the diameter of 5 cm, where the interval of 5 cm from different soil layers was used to determine the content of soil nuclide. A gamma spectrometer was used to measure the 137Cs content, and further to calculate the area specific activity. A wet sieving was utilized to obtain the aggregates with four particle sizes (>2 mm, 2~0.25 mm, 0.25~0.053 mm and <0.053 mm). The approach of heating on a potassium acid water-bath was adopted to obtain the content of organic carbon in the aggregates. One-Way ANOVA, DMRT and redundancy analysis operations were used to examine the correlation between content of soil aggregate organic carbon and physical and chemical properties. The results found that the top layer of gully erosion had the highest 137Cs content, even higher than the background value, indicating the sedimentary area, whereas, the upper slope and the lope toe were moderately eroded, while the slope mildly was eroded. The evolution of gully erosion area successively went through the absolute deposition, absolute erosion, relative deposition, absolute erosion in a top-down way along the slope, in which the vegetation and terrain were the dominant factors. The content of large aggregates in the sedimentary area was significantly higher than that in the eroded area, whereas, the eroded area had significantly higher content of microaggregates, whose components continued to increase as the erosion intensity strengthened. Moreover, the soil aggregates in the sedimentary area had a higher stability, whereas, the lowest stability occurred at the mostly eroded toe of slope. Compared with the upper and toe slope, the soil aggregates in the middle slope of erosion area showed higher values of macroaggreages content and mean weight diameter. The content of organic carbon in soil aggregates for all particle sizes was higher in the sedimentary area than that in the erosion area. There was also uniform distribution of organic carbon in soil aggregates in the erosion area. The content of organic carbon in the large aggregates was significantly higher than that in the microaggregates. The correlation analysis results showed that the organic carbon of only 2-0.25 mm aggregate contributed positively to aggregate stability (P<0.05), and the correlation coefficient was 0.75. It infers that the content of organic carbon in the microaggregates was positively correlated with the increase of erosion intensity. Compared with traditional slopes, the top of gully erosion area was a depositional site, but other slopes were erosional sites. The soil structure and related carbon components were mainly affected by the degree of soil erosion under the domination of terrain and vegetation. Therefore, the specific measures of water and soil conservation if appropriate can be taken in special terrains in red soil areas of southern China.