Mineralization and nitrification potentials of grassland soils at shallow depth during laboratory incubation

In grassland ecosystems when ample organic N is present, both, mineralization and nitrification can play an important role in determining fertilizer use efficiency as well as N losses to the environment. Laboratory incubation studies were undertaken in a controlled environment at 20 °C to establish relative potential rates of nitrification and mineral N variation in soil collected from grassland fields. Soil samples of 0 - 2.5, 2.5 - 5.0, and 5.0 - 7.5 cm were collected to examine the depth distribution of mineral N. Mineralization potential was determined from soil without added N while nitrification activity was measured following the addition of NH4+-N during 42 days period. Net mineralization of N ranged from 13 to 64 mg (kg soil) - 1. Of the total inorganic N found, more than 50 % was released from the surface 0 - 2.5-cm and the concentration decreased with depth. In a separate experiment when 15N was used, net mineralization was dominant over immobilization and of the gross mineralization, more than 70 % was released as inorganic N. Nitrification showed an initial lag phase, a maximum rate phase and a reduced rate phase. The maximum rate of nitrification ranged from 3.3 to 7.5 mg (kg soil) - 1 day - 1 being greatest in the 0 - 2.5 cm depth. The nitrification rate decreased and the delay phase increased with soil depth. During the study, 50 to 60 % of added NH4+-N was converted into NO3 - -N indicating the presence of active nitrifiers and a large potential for nitrification in the soil. A significant variation in nitrification and mineralization rates within 0 - 7.5 cm was observed which is extremely important in transformations and dynamics of N in grassland ecosystems.