Biocrusts （biological soil crusts ）, as ubiquitous living covers on soil surface of the revegetated grasslands in hilly regions of the Loess Plateau, are potential influence factors on the availability of soil P （phosphorus）. However, researches focus on the relationship between soil P availability and development of biocrusts were still fresh so far. Impacts of biocrusts on soil total P, available P and phosphatase activity were investigated in revegetated grasslands at different stages of biocrusts developed after the survey of biocrusts distribution. The objects of the study were to determine the influences of biocrusts development on the availability of soil P in the region. The results show that the biological crusts could significantly improve soil total phosphorus contents in the biocrusts layers, while the contents of total phosphorus in 0-10 cm layers are not significant. Soil available phosphorus contents of the biocrusts in the study area are in the range of 3.27-5.87 mg/kg, which are accounting for O. 57%-0. 95% of total P in the same layer. The alkaline phosphatase activities in biocrusts are significantly higher than those of the lower soils （0-10 cm） 3-81 times. The effects of biological crusts on the availability of soil P and phosphatase activity are related to the developmental stages of biocrusts. The biological crusts increase availability alkaline phosphatase activity and organic matter, of soil phosphorus. These results suggest that and reduce the crusts on soil pH, finally improve the availability of soil P could be significantlyenhanced for the formation of biocrusts.%黄土丘陵区生物结皮广泛发育，可影响土壤磷素有效性。目前鲜见生物结皮对土壤磷素有效性的研究报道。本文以该区不同年限退耕地的生物结皮为研究对象，通过野外调查和室内分析，研究了生物结皮对土壤全磷、有效磷及碱性磷酸酶活性的影响。结果表明，1）生物结皮的形成可显著提高结皮层土壤全磷含量，而下层（0-10121／1）全磷含量差异不显著；2）生物结皮的形成可显著提高结皮层土壤有效磷含量，研究区生物结皮层土壤有效磷含量为3．27-5．87mg／kg，占到同层土壤全磷含量的0．57％-0．95％，生物结皮层磷酸酶活性高于下层（O-10em）3-81倍；3）生物结皮对土壤磷素有效性及碱性磷酸酶活性的影响与生物结皮发育阶段有关；4）生物结皮主要通过提高结皮层土壤碱性磷酸酶活性和有机质含量，降低土壤pH，进而提高了土壤磷素有效性。本文研究结果表明，生物结皮的形成有助于提高黄土丘陵区退耕地土壤磷素有效性。
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