The increase of atmospheric nitrogen (N) deposition caused by global change and industrial and agricultural production has had an important impact on the structure and function of ecosystems. There are many forms in composition of atmospheric N deposition. However, it is not clear whether there are differences in the effects of N deposition forms on structure and function of the ecosystems. Here our objective was to characterize the effects of different forms and levels of N addition on soil net N mineralization potential of steppe ecosystem in the Nei Mongol. Methods
A N addition experiment was carried out in the meadow steppe in Nei Mongol using five different N fertilizers, including CO(NH2)2, NH4HCO3, NH4NO3, (NH4)2SO4, and slow-release urea separately since 2014. There were six N addition levels with 0 (N0), 2 (N2), 5 (N5), 10 (N10), 20 (N20) and 50 (N50) g·m -2·a -1. Fresh soil samples from all treatments were taken and all roots were removed in July 2016. Then these soil samples were incubated for 24 h at 25 °C with 60% field water capacity. The potential of net N mineralization and nitrification rates and the potential of soil microbial respiration (MR), soil physical and chemical properties, soil microbial biomass carbon (MBC) and N (MBN) contents were measured, respectively. Important findings
The results showed that: (1) different forms and levels of N addition significantly increased soil inorganic N content and potential net N mineralization and nitrification rates. The N20 treatment had the highest soil inorganic N content and net N mineralization rate, however the highest soil net nitrification rate was found under N50 treatment; (2) different forms and levels of N addition significantly increased MBC and MBN contents and decreased the microbial metabolic quotient (qCO2). Lower N addition (N2) enhanced MR, but medium and higher N addition (N20, N50) restrained the MR; (3) different forms and levels of N addition significantly reduced the soil pH value, but significantly increased the available phosphorus content. No effects were found in soil water content, total phosphorus, total N and soil organic carbon contents, separately. The results verified that soil available N was the limited factor affecting plant productivity in meadow steppe in Nei Mongol steppe. No matter what type of N fertilizer can increase the activity of soil microorganism and the potential net N mineralization rate of the meadow steppe in this area.