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.