Characteristic is and evolution of soil nitrogen in dam land and terrace in Loess Hilly Region

【Objective】 The characteristics and evolution of soil nitrogen under different use years in dam land and terrace in loess hilly region were studied to provide scientific instructions to evaluate exactly the supply potential of soil nitrogen and fertilize reasonably nitrogenous fertilizer.【Method】 Soil samples of 0-20 cm,20-40 cm and 40-60 cm under different use years in dam land and terrace were taken to test the contents of organic matter(OM),total nitrogen(TN),available nitrogen(AN) and mineral nitrogen(NO3——N and NH4+-N) by using general methods.The characteristic,variability and temporal and spatial trend of these nutrient indicators were studied systematically by using statistical analytical method.【Result】 The spatial variability of soil nitrogen in topsoil of dam land and terrace was large.Only average content of AN of terrace was at the medium level among average content of OM,TN,and AN in topsoil of dam land and terrace.Average content of OM belonged to lower level and average content of TN and AN was at the low level in 0-60 cm profile of dam land and terrace.NO3——N/NH4+-N of dam land and terrace was more than 1,namely,NO3——N was main body of AN.The interrelations of OM,TN,AN,NO3——N and NH4+-N were very significant with each other,and their contents were affected to some extent by particle composition of soil,aspect and position of the slope.Significant differences(P0.05) between contents of OM,TN,AN,NH4+-N and initial mineral nitrogen of to psoil and that of 20-40 cm and 40-60 cm in dam land and terrace were found,but their content between 20-40 cm and 40-60 cm has no significant difference,and their average content of 0-60 cm in dam land were larger than that of terrace respectively.The contents of OM,TN,AN and NO3——N/NH4+-N increased incipiently and decreased subsequently with the increase of dam land and terrace use years.The critical year(39-42 years) of the degradation for soil nitrogen in dam land hung behind 13 years at least than that of terrace(22-29 years).【Conclusion】 As a whole,soil nitrogen of dam land and terrace was poor in loess hilly region.Current supply ability of soil nitrogen in dam was higher and its potential capacity was more durative and steady than that of terrace.