The different characteristics of soil fixed ammonium from south to north on the Loess Plateau

The total soil nitrogen,fixed ammonium and their relationship among the Ust-Sandiic Entisols(Shenmu),Los-Orthic-Entisol(Yan'an) and Eum-Orthic Anthrosol(Yangling) on the Loess Plateau were investigated to assess the influences of soil location and depth(0-200 cm) on their contents and to accumulate first-hand data of soil nitrogen accumulation on the Loess Plateau.The results showed that the contents of total soil nitrogen and fixed ammonium differed significantly with location and soil depth.The contents of soil total nitrogen and fixed ammonium decreased significantly from south to north on the Loess Plateau,but their distributions differed significantly.In the same location,the content of total soil nitrogen decreased with soil depth remarkably within 0-60 cm,slightly within 60-120cm,and kept stable under 120 cm,whereas the content of soil fixed ammonium had no obvious change in the profiles.The average content of soil fixed ammonium was 215.807.45,165.808.73 and 146.501.83 mg/kg within 0-10 cm for EumOrthic Anthrosol,Los-Orthic-Entisol and Ust-Sandiic Entisols,respectively,and the averages of them for the three soils were 193.409.67,157.145.75 and 142.025.47 mg/kg within 10-200 cm,respectively.The average soil fixed ammonium nitrogen was(39.570.78)%,(32.916.82)% and(29.747.01)% of total nitrogen within 0-10 cm,and(89.5213.42)%,(59.5213.86)% and(47.269.01)% of total nitrogen within 10-200 cm of Ust-Sandiic Entisols,Los-Orthic-Entisol and Eum-Orthic Anthrosol,respectively.There was positive relationship between the contents of physical clays and soil fixed ammonium(P0.01),indicating that physical clay was the primary carrier of soil fixed ammonium.There was also positive relationship between the content of soil organic nitrogen and fixed ammonium(P0.01).The results showed that the content of soil fixed ammonium kept stable,whereas,soil total nitrogen differed with soil depth in the profiles at the same location.This characteristic was an inevitable result of Loess Plateau formation and supported the eolian theory of loess parent material.The results also indicated that the traditional understanding of the percentages of organic nitrogen in total nitrogen and the status of fixed ammonium in total nitrogen should be reevaluated.