Abstract: The soil longitudinal sections of different plant populations during different stages of vegetation succession show that the urease activities are more intense in the upper than in the under layers. The activities abate slowly but evenly, with notable differences layer by layer. The highest content of surface soil urease activity reaches 186.4 mg·kg^-1h^-1,while the lowest content amounts to 72.1 mg·kg^-1h^-1. With the deve-loping vegetation succession, the activities of the soil enzymes gradually increase, while the ratio of the surface enzymes activities decrease. Urease activities in the natural soil show a positive correlation with organic matter, total nitrogen, total phosphorus, and available nitrogen, a significant positive correlation with available potassium, and a significant negative correlation with calcium carbonate (CaCO₃). The result of the path analysis shows that the direct acting coefficient of soil chemical and physical factors to the soil urease activity is in the following order:Total N>CaCO₃>Organic matter>Soil pH>Total phosphorus. The soil total nitrogen tops all the other factors in the direct acting coefficient and enacts an indirect coefficient to the urease activity through the organic matter and total phosphorus, indicating that it affects both directly and indirectly to the urease activities. The organic matter does not effect directly on the urease activity, but indirectly through total nitrogern and total phosphorus, which means that organic matter enacts only an indirect effect to the urease activity. Other factors like total phosphorus, available nitrogen, and cation exchange capacity effect only limited direct or indirect coefficient on the urease activities. They are not factors affecting the soil urease activity.

Key words: Urease activity, Soil properties, Path analysis, Loess hilly region

摘要： 不同演替阶段植物群落土壤剖面中脲酶的变化趋势是上层明显高于下层,土壤脲酶活性下降较为缓慢均匀,层次之间差别明显。表层脲酶含量最高的186.4mg·kg^-1h^-1,最低的72.1mg·kg^-1h^-1。随着植被的演替发展,酶活性逐步提高,表层所占比例逐步降低。土壤脲酶活性与有机质、全氮、全磷、速效氮呈极显著正相关,与速效钾呈显著正相关,与碳酸钙呈显著负相关。通径分析结果表明:土壤理化因子对脲酶活性直接作用系数的序次为全氮>碳酸钙>有机质>土壤pH>全磷;土壤全氮对脲酶活性的直接通径系数最大,而且它通过有机质和全磷对脲酶的间接效应也很明显,说明它对脲酶活性具有强烈的直接效应和间接效应。有机质对脲酶活性的直接通径系数较小,但有机质通过全氮、全磷对脲酶活性的间接系数却很大,对脲酶活性的影响主要是间接作用;全磷、速效氮和CEC等对脲酶活性直接通径系数都较小,而且通过其它因素对脲酶活性的间接通径系数之和也较小,不是影响土壤脲酶活性的主要因素。

关键词: 脲酶活性, 土壤性质, 通径分析, 黄土丘陵区