以黄土丘陵区易形成生物结皮的阴坡自然撂荒地为研究对象,运用时空互代法,通过野外调查结合室内分析,研究了不同年限(1～13a)撂荒地上生物结皮的形成对0～20cm剖面土壤含水量、容重、地表粘结力、水分渗透性、有机质含量以及土壤全量和速效养分含量等基本理化性状的影响。结果表明,生物结皮的形成可显著影响0～20cm土层土壤的理化性状,表现为:①退耕后,由于生物结皮的形成,退耕地较农地土壤渗透性显著降低,0～20cm土层土壤含水量随退耕年限的延长经历了一个先增后减的变化过程,导致了10a以上退耕地土壤含水量的降低;②生物结皮可显著提高退耕地土壤表面粘结力,10a以上退耕地保留结皮土壤的粘结力是初始退耕地和/或除掉结皮层土壤粘结力的6倍以上,退耕地表面粘结力的增强与退耕后土壤自然沉实以及容重增加关系不大;③生物结皮的形成导致了退耕地土壤渗透性的降低,保留结皮的土壤,其稳定渗透系数(K10)随退耕年限延长而降低,除掉结皮,K 10随退耕年限延长而增加;④生物结皮的形成,使土壤有机质、全氮含量剖面分布呈表聚现象,随退耕年限的延长,表聚现象更趋明显,土壤全磷亦有弱表聚现象。生物结皮层的形成,还可提高结皮层土壤速效养分含量。

Biological soil crust plays an important role in degraded ecosystem restoration.Large area of biological crust has formed after cropland shifting to revegetated grassland on the hilly Loess Plateau of China. Our objective was to determine the impact of biological crust on soil chemical and physical properties in the study area.In this paper,through field investigation and sampling analysis,some physical and chemical properties of different years rehabilitated grasslands on the northern slopes which had biological soil crust formed on the surface were studied.Eight research sites were selected and soil and crust samples were collected in April 2004,with the restriction that locations were confined to a single soil series that was representative of the local environmental condition,similar elevation and slope.At each sampling site,five samples taken randomly were mixed to form a composite soil sample.Samples were taken from the 0 to 20 cm deep profile in 6 layers(crust,0-2,2-5,5-10,10-15 and 15-20cm).Soil moisture,bulk density,cohesion of soil surface,infiltration rate,organic matter content,total nitrogen,total phosphorus and available N,P,K contents of the 0-20cm soil profile were measured for soil samples taken from the 8 sites.The results showed soil physical and chemical properties of the 0-20cm profile were affected significantly by the biological crust.1)After shifting from cropland to revegetated grassland,soil moisture in the 0-20cm profile increased firstly and then decreased with the increase of rehabilitation years.Because the formation of the biological crust,soil penetrability of the rehabilitated grasslands reduced significantly compared with cropland,this also resulted in lower soil moisture in this profile after 10 years' rehabilitation.2)Soil surface cohesion of the rehabili-tated grassland increased significantly with the formation of the biological crust after shifting from cropland.After 10 years revegetation,the crusted soil surface cohesion of the revegetated grassland was 6 times that of the fresh revegetated grassland.The reason for this was mainly due to the formation of the biological crust,rather than the increase of soil bulk density.3)Soil penetrability reduced significantly when shifting from cropland to revegetated grassland.The infiltration coefficient of crusted soil decreased gradually with the increase in years of revegetation.However,the infiltration coefficient increased gradually when getting rid of the crust.4)The formation of the biological crust brought to an enrichment of soil organic matter and total nitrogen in the upper soil.With the increased years of rehabilitation,the enrichment was even more obvious,and soil total phosphorus was also enriched to some extent.Soil available nutrient was improved as a result of the formation of the biological crust.