Heterogeneity of soil nutrients of cropland returning to forest on the loess hilly region

Typical cropland returning to forest with different fixed number of years under different vegetation types in Yan'an,Ansai and Wuqi on the loess hilly region were selected as experimental sites to study the heterogeneity of soil nutrient.Soil samples were collected using S shape method and analyzed using soil nutrient systematic approach.Soil nutrient variability was studied with traditional statistics method.The results showed that soil nutrient content manifested remarkable variability under different vegetation types.Soil organic matter,total N,available N and available K had a relatively greater variability,which were mainly attributed to the former agricultural fertilization system.However,total P and available P had a smaller variability.Except total P and available P,the content and variability of soil nutrients with different fixed number of years returning cropland to forest showed remarkable discrepancy.In the beginning of cropland returning to forest,the content and variability of soil organic matter,total N,available N and available K were higher caused by tillage and fertilization.Vegetation index showed an ascending tendency with a great variability.Thereafter,soil nutrients took on a declining process in company with the quit of cultivation and fertilization and the consumption of soil nutrients by vegetation restoration.Soil nutrients had a great variability as well.Along with further progress of vegetation restoration and continue of community succession,litter fall of vegetation successively increased and soil progressive developed too.Both vegetation index and soil nutrients showed an increasing tendency and got stable state around 20 years of cropland returning to forest.However,soil development lagged behind change of vegetation characteristics.