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 nitrification inhibitor may be advantageous for nitrogen (N) fertility management for soft red winter wheat (Triticum aestivum L) production in Missouri. The objective of this study was to determine the duration of NH4+ retention in the soil during fall and winter from anhydrous ammonia (NH3) with and without nitrapyrin [2-chloro-6-(trichloromethyl) pyridine] applied just prior to establishment of winter wheat. Anhydrous ammonia was applied with and without nitrapyrin in a Mexico silt loam (fine montmorillonitic, udollic ochraqualf). Three days after application 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 both years. The decline of these NO3- levels to background concentrations may have been due to leaching and plant uptake. There was a relationship between declining NH4- levels and accumulating growing degree days (GDD) in both treated and untreated plots. Nitrate levels in the soil profile increased during the first three weeks of the study, but levels decreased following wheat establishment and remained relatively low during the remainder of the sampling period in both years. Nitrate levels were lower in soils from plots treated with nitrapyrin than in untreated plots. A negative relationship was observed between NO3- levels 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 relationship was observed between NO3- levels and GDD in plots treated with nitrapyrin during the 1991-92 sampling period. Nitrapyrin improved NH4+ retention in the soil profile during fall and winter of both years.