[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, NH4(+) -N, NO3(-) -N, NO2(-) -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, NH4(+)-N, NO3(-)-N, NO2(-)-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, NO2(-) -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.