Responses of soil nitrogen mineralization and nitrification to reclamation years and modes of coal mine

Soil nitrogen mineralization and nitrification are impacted by microbial activity and soil properties. The reclaimed coal mine soil are artificially reconstructed soil with different properties from farmland soil, such as lower organic matter content, small nitrogen sink, weak microbial activity. In order to reveal the response of nitrogen mineralization and nitrification of reclaimed soils in coal reclamation areas and to explore the soil nitrogen conversion efficiencies under different reclamation patterns and for different reclamation years, a laboratory experiment with soil sampled from the reclaimed coal mine were conducted. There were six sampling soils in Antaibao Opencast Mine, Shanxi Province, which were unreclaimed soil (CK); naturally reclaimed soil for 3 years (NRL); alfalfa pasture soils reclaimed for 3 years (AL3), 9 years (AL9), 21 years (AL21); and buckwheat soil reclaimed for 3 years (BL3). Nitrogen mineralization and nitrification of sampled soils were measured by using Stanford aerobic incubation method and constant temperature culture. And then, the first-order reaction kinetics model and Logistic equation were used to fit the process of organic nitrogen mineralization and nitrification. The results showed that the mineralization rate was highest in alfalfa field reclaimed for 3 years (AL3), and lowest in alfalfa field reclaimed for 21 years (AL21). The mineralization of nitrogen was rapid in the first 7 days and gradually became gentle, tended to be stable in 28 days of incubation. The first-order kinetic equations suggested that nitrogen mineralization potential ranged from 89.28 to 124.51 mg·kg^-1 and was in the order of AL21 > NRL > AL3 > BL3 > CK > AL9. The mineralization rate constant ranged from 0.022 6 to 0.051 9, with the order of AL3 > AL9 > CK > NRL > BL3 > AL21. There was a significantly positive correlation between nitrogen mineralization potential and soil organic matter content (r=0.91). The soil nitrogen capacity was significantly increased in the long-term reclamation of grassland, and the mineralization process was more stable. The change of nitrate content in various soils of reclamation area during incubation was roughly following a "S" trend and was divided into three stages, which were the early stage (0-5 d), the rising stage (5-14 d) and the stable stage (14-28 d). Logistic equation fitting results showed that the reclamation years significantly affected the peaking time of nitrification (the max difference was 6.85 days among different reclamation years of alfalfa pasture). The drastic nitrification process in alfalfa pasture reclaimed for 21 years was intense and short, that of naturally restored soil for 3 years was slow and long. The arable land had a greater nitrification rate and longer nitrification time than grassland. In summary, under long term plantation of alfalfa in reclaimed coal mine soil, soil nitrogen sink increased, nitrogen mineralization became stable.