Greater Recovery of Fertilizer Nitrate Vs Ammonium Sources in Agricultural Soils of Central Montana.

Winter wheat (Triticum aestivum L.) recovery of spring applied N fertilizer sometimes is greater from NO₃–N than NH₄–N sources in central Montana. We conducted laboratory and field experiments on a Tamaneen clay loam (fine, smectitic, frigid Typic Argiustolls) to examine the hypothesis that NH₄-N based fertilizers are preferentially immobilized relative to NO₃-N forms and to investigate the fate of fertilizer ¹⁵N applied to a no-till winter wheat crop. Short-term immobilization of added Na¹⁵NO₃ and (¹⁵NH₄)₂SO₄  was determined within a gradient of 0, 10, or 100 mg of glucose-C over a laboratory incubation period (26 d) at 10 °C. Fertilizer N immobilization (FNI) vs. incubation-day relationships conformed to an exponential rise to maximum functions. Cumulative FNI was equivalent to 0.8, 27.6, and 65.9% for NaNO₃ and 8.8, 41.2, and 65.3% for (NH₄)₂SO₄ at 0, 10, and 100 mg glucose-C, respectively. Fertilizer N immobilization ratios, FNI_(NH4)2SO4:FNI_NaNO3, averaged (1–26 d) 10.7, 1.8, and 1.5 for 0, 10, and 100 mg glucose-C, respectively. In the field, ¹⁵N labelled fertilizer (urea, urea+NBPT, and NaNO₃) was applied in the late fall, winter, and early spring (100 kg N ha^-1). The amount of ¹⁵N immobilized in soil (0-15 cm) was equivalent to 26.2, 23.0, and 12.7 % for spring applied urea, urea+ NBPT, and NaNO₃, respectively. Fertilizer ¹⁵N recovery by winter wheat (grain + straw) was greater for the early spring (55.5 %) compared to late-fall (40.2 %) and winter (40.9 %) applications. More fertilizer ¹⁵N was recovered in the case of NaNO₃ (57.1 %) than for urea (37.4 %) and, urea+NBPT (42.1 %). This study confirmed our hypothesis that preferential immobilization of NH₄-N compared to NO₃-N can lead to greater recovery of NO₃-N vs. NH₄-N sources in agricultural soils of the northern Great Plains.