吴起县退耕还林对土壤团聚体水稳性和养分含量的影响

Effects of the Returning Farmland to Forests on Content of Water Stable Soil Aggregates and the Nutrients in Wuqi County

土壤团聚体组成及其稳定性在土壤质量和抗蚀性评价中具有重要意义（张治伟等，2009）。土壤退化首先表现出团粒结构的破坏与消失（王晓娟等，2012），在黄土高原常将大于0．25mm水稳性团聚体的质量百分含量作为土壤抗蚀性指标。

The percentage of water stable aggregates of different sizes and nutrients content in topsoil layers under different vegetation types in Wuqi County were measured to analyze the effects of the returning farmland to forests on water stable aggregates and nutrients. Results showed percentage of soil water stable aggregates with the diameter 〉 0.25 mm, mean weight diameter （MWD） of water stable aggregates and soil nutrients from forests and uncultivated land were greater than those from farmland. However, the fractal dimensions （D） of size distributions of soil water stable aggregates from farmlands were greater. The results indicated that the returning farmland to forests could make soil structure more stable and soil nutrients increase significantly. There were significant relationships between organic matter, available phosphorus and MWD, percentage of soil water stable aggregates with the diameter 〉 0.25 mm and D. Moreover, the content of organic matter in the water stable aggregates with the diameter less than 0. 075 mm was 12.70 g·kg^-1, which was least. While the content of organic matter in the water stable aggregates with the diameter larger than 0.25 mm was relatively larger, with 39.66 g·kg^-1. Thus, soil organic matter existed mainly in the larger soil water stable aggregates, suggesting that soil organic matter would provide a good material basis for the formation of larger soil aggregates. Multivariate statistical analysis showed content of soil nutrients was higher and soil structure was more stable in the forest plots with older forest age and on sunny slope in the research area.