NITRIFICATION AND COUPLED NITRIFICATION-DENITRIFICATION ASSOCIATED WITH A SOIL-MANURE INTERFACE

Nitrification and denitrification in soil often occur in close proximi ty so that a substantial part of the NO3- formed by nitrification afte rwards diffuses to the anaerobic denitrification zone where it is redu ced to N-2. The stable N isotope based isotope pairing method was used to study nitrification and coupled nitrification-denitrification asso ciated with a manure hot-spot in soil contained within a diffusion cha mber. During the 3-wk incubation period, a very active nitrification z one developed within 0.7 to 1 mm from the aerobic-anaerobic soil-manur e interface, and within this thin zone, all NH4+ liberated from the ma nure was oxidized. The short distance between the nitrification zone a nd the anaerobic manure facilitated coupled nitrification-denitrificat ion, which turned out to be the main sink for Liberated inorganic N wi th 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 applica bility of the isotopic pairing method was verified by using variable c oncentrations of (NO3-)-N-15 and by selectively inhibiting nitrificati on with acetylene. The change in (NO3-)-N-14 profile following additio n of acetylene was simulated by a diffusion reaction model to obtain e stimates of the underestimation of denitrification by use of the commo nly used acetylene blockage technique. incubation periods with acetyle ne of 1, 3, 6, and 12 h reduced the coupled nitrification-denitrificat ion rate to 46, 25, 16, and 10%, respectively, of the original rate, a nd use of the acetylene technique for determining denitrification in m anure-amended soils thus cannot be recommended.