RETENTION OF NITROGEN FROM STABILIZED ANHYDROUS AMMONIA IN THE SOIL-PROFILE DURING WINTER-WHEAT PRODUCTION IN MISSOURI

Delaying nitrification of ammonium (NH4+)-based fertilizers with a nit rification inhibitor may be advantageous for nitrogen (N) fertility ma nagement for soft red winter wheat (Triticum aestivum L) production in Missouri. The objective of this study was to determine the duration o f NH4+ retention in the soil during fall and winter from anhydrous amm onia (NH3) with and without nitrapyrin [2-chloro-6-(trichloromethyl) p yridine] applied just prior to establishment of winter wheat. Anhydrou s ammonia was applied with and without nitrapyrin in a Mexico silt loa m (fine montmorillonitic, udollic ochraqualf). Three days after applic ation of NH3, soil samples were collected from the injection zone (IZ) and the surrounding profile up to 8 cm from the IZ. Sampling occurred weekly or biweekly until wheat harvest. Ammonium levels in the IZ and the surrounding profile decreased during both the 1991-92 and 1992-93 growing seasons, but the decline was slower in treated plots. Higher levels of NO3- accumulated in the IZ during the first four weeks of bo th years. The decline of these NO3- levels to background concentration s may have been due to leaching and plant uptake. There was a relation ship between declining NH4- levels and accumulating growing degree day s (GDD) in both treated and untreated plots. Nitrate levels in the soi l profile increased during the first three weeks of the study, but lev els decreased following wheat establishment and remained relatively lo w during the remainder of the sampling period in both years. Nitrate l evels were lower in soils from plots treated with nitrapyrin than in u ntreated plots. A negative relationship was observed between NO3- leve ls and GDD in untreated plots in 1991-92 (R-2=0.30) and also In 1992- 93 in both treated (R-2=0.67*) and untreated (R-2=0.62**) plots. No r elationship was observed between NO3- levels and GDD in plots treated with nitrapyrin during the 1991-92 sampling period. Nitrapyrin improve d NH4+ retention in the soil profile during fall and winter of both ye ars.