Nitrification Rates in Profiles of Differently Managed Soil Types

The variation of nitrification rates among soils and within soil profiles has received little attention compared with the well‐established relations with soil moisture and temperature. In this study, the rate of nitrification was determined in profiles of various soil types and under different agricultural management. Verhulst's equation was used to express the accumulation of nitrate (NO ‐ 3 ) with time and modified to describe the rate of ammonium (NH + 4 ) decrease. The maximal rate of nitrification ( K mx ) and the delay period ( t ′) were derived from the equation and used to characterize quantitatively the nitrification process in various soil samples. The K mx and t ′ of surface soil samples were from 5 to 70 mg kg −1 d −1 and from 0.2 to 8 d, respectively. The K mx decreased and t ′ increased with soil depth. Soil factors mostly affecting the rate parameters were: pH decreasing from 7.8 to 6.6 or HCO ‐ 3 decreasing below 1 mol m −3 , previous agricultural management, and soil depth. Data obtained from 15 N studies indicated that mineralization rate of soil organic N in NH + 4 ‐treated soils was not always negligible relative to nitrification.