Nitrification and coupled nitrification-denitrification associated with a soil-manure interface

Nitrification and denitrification in soil often occur in close proximity so that a substantial part of the NO3- formed by nitrification afterwards diffuses to the anaerobic denitrification zone where it is reduced to N-2. The stable N isotope based isotope pairing method was used to study nitrification and coupled nitrification-denitrification associated with a manure hot-spot in soil contained within a diffusion chamber. During the 3-wk incubation period, a very active nitrification zone developed within 0.7 to 1 mm from the aerobic-anaerobic soil-manure interface, and within this thin zone, all NH4+ liberated from the manure was oxidized. The short distance between the nitrification zone and the anaerobic manure facilitated coupled nitrification-denitrification, which turned out to be the main sink for Liberated inorganic N with maximum rates of 140 nmol N-N-2 cm(-2) h(-1), thereby removing 90% of the Inorganic N liberated during the incubation period. The applicability of the isotopic pairing method was verified by using variable concentrations of (NO3-)-N-15 and by selectively inhibiting nitrification with acetylene. The change in (NO3-)-N-14 profile following addition of acetylene was simulated by a diffusion reaction model to obtain estimates of the underestimation of denitrification by use of the commonly used acetylene blockage technique. incubation periods with acetylene of 1, 3, 6, and 12 h reduced the coupled nitrification-denitrification rate to 46, 25, 16, and 10%, respectively, of the original rate, and use of the acetylene technique for determining denitrification in manure-amended soils thus cannot be recommended.