NITRIFICATION OF AMMONIUM NITROGEN IN SOILS TREATED WITH XDE-474

Retardation of NH4+ nitrification constitutes one means to conserve fe rtilizers applied to soils. A new nitrification inhibitor, XDE-474, ha s been developed by DowElanco Chemical Company (Indianapolis, IN) and needs to be evaluated. Laboratory studies evaluated the effect of nitr apyrin [2-chloro-6-(trichloromethyl)pyridine] and XDE-474 on nitrifica tion rates in five Iowa soils varying markedly in organic C (OC), pH, and texture. A 10-g sample of surface soil treated with 1 mL of soluti on containing 2 mg N as either NH4OH or (NH4)(2)SO4 and 4 mu g a.i. g( -1) soil of nitrapyrin or XDE-474 was incubated at 20 +/- 1 degrees C for 0, 7, 15, 30, 45, and 60 d. First-order equations were used to cal culate the maximum nitrification rate (K-max), duration of lag period (t'), period of maximum nitrification (Delta t), and duration of lag p lus maximum nitrification period (t(s)). Nitrification rates were sign ificantly affected by N sources and inhibitors used. In the Okoboji so il (43.0 g kg(-1) OC), nitrification of NH4OH was 19.5 mg kg(-1) d(-1) NO3--N in the control soil but reduced to 15.2 or 4.43 mg kg(-1) d(-1 ) NO3--N when nitrapyrin or XDE-474 were added, respectively, Extensio n of the lag period due to the inhibitors varied from 0.73 d in the Ok oboji soil to 18.8 d in the Storden soil (11.0 g kg(-1) OC). Multiple regression equations expressing the dependence of K-max on OC, pH, and clay showed that only OC and pH were significant in accounting for th e variation in K-max. The resulting three-dimensional response surface indicated that XDE-474 was less affected than nitrapyrin by changes i n OC and pH.