[Variation of soil nitrogen during in situ mineralization process under different grasslands in the mountainous area of southern Ningxia, Northwest China].

Variations in organic nitrogen,microbial biomass nitrogen,soluble organic nitrogen,NH+4-N,NO-3-N,NO-2-N and N mineralization were investigated under three different grasslands in the mountainous area of southern Ningxia,Northwest China( natural grassland,artificial turf and abandoned land) using the close-top tube incubation method. Microbial biomass nitrogen,soluble organic nitrogen,NH+4-N,NO-3-N,NO-2-N and N mineralization exhibited significant seasonal variations. The nitrogen levels remained essentially unchanged from April to June,significantly decreased in July-August,rebounded after August,and were lowest in August.The organic nitrogen content remained unchanged in the whole training process. The soil mineralization rates,nitrification and ammonification rate were lowest in June-August. The ratios of each N fraction to total N responded differently to seasonal changes. The ratios of organic N,NO-2-N to total N did not change,in contrast,the ratios of nitrate N,microbial biomass nitrogen and soluble organic nitrogen,to total soil N decreased from April to August,and increased from August to December. The soil organic matter,pH,and bulk density were closely related to soil N. There was a significant positive correlation among the six N fractions. The soil nitrogen content of the different grassland types followed the order of natural grassland abandoned land artificial turf.