Gross nitrogen transformations in black soil under different land uses and management systems

In this study, the N-15 isotope pool dilution technique was used to investigate the gross N transformation rates in black cropland soils that had been subjected to different fertilization treatments [no fertilizer application (NF), N and P applications (NP), and N and P applications with composted pig manure (NPM)] compared to a neighboring grassland soil in northeast China. The results showed that gross N mineralization and immobilization rates were significantly higher in grassland soils than in cropland soils, whereas the reverse was true for gross nitrification rates, which led to a higher gross nitrification rate/ammonium immobilization rate in cropland soils compared to grassland soils. Long-term applications of chemical fertilizer increased the gross rates of mineralization, immobilization, and nitrification compared to the no fertilizer treatment, and this effect was more prominent when combined with application of organic manure. However, the ratio of gross nitrification rate to ammonium immobilization rate in the NPM treatment decreased compared to the NP and NF treatments, which suggested that converting grassland to cropland reduced the potential release and conservation of N and promoted N loss via NO3- leaching, whereas appropriate management practices, such as the combined application of chemical fertilizer and pig manure, could increase N retention and decrease the production and leaching of NO3-