Microbial Immobilization of 15N Labelled Ammonium and Nitrate in an Agricultural Soil from Central Montana.

Field studies in central Montana have revealed greater fertilizer N recoveries in wheat of N applied as NaNO₃ compared to urea.  This study determined whether soil microbial immobilization of fertilizer N was affected by the N form (NH₄-N vs. NO₃-N) for an agricultural soil from central Montana amended with glucose-C. A lab incubation study was conducted at 10˚C over 26 d on a Tamaneen clay loam and consisted of a factorial of two N sources and three C levels.  Soil (20 g) was spiked with 2 mg N as ¹⁵N-labelled (98 atom %) NaNO₃ and (NH₄)₂SO₄, and a gradient of immobilizing conditions was imposed through the addition of 0, 10, 100 mg glucose-C.  Soil heterotrophs immobilized both N sources and immobilization rates increased with the addition of glucose-C. With the addition of 100 mg C, inorganic N was depleted quickly in the soil treated with (NH₄)₂SO₄ and dropped to < 0.1 mg N kg^-1 after 5 d. For the soil treated with NaNO_3, inorganic N decreased slowly to 6.2 mg kg^-1 by 11 d and remained at, or near, this level through the remainder of the incubation period. At 15 d,   the ¹⁵N isotopic ratio in the organic-N fraction was larger for (NH₄)₂SO₄ compared to NaNO₃ at 0 and 10 mg but similar at 100 mg glucose-C (N source x Carbon  P <0.0001).  The percentage of applied N immobilized was 0.80, 26.38, and 65.27% for NaNO₃ and 7.38, 47.34, and 60.78% for (NH₄)₂SO₄ at 0, 10 and 10 mg glucose-C, respectively. Soil heterotrophs abundance, cultured in Petri dishes, was enhanced with increasing C availability especially in soils treated with (NH₄)₂SO_4.   Greater recovery by wheat of NO₃-N compared to NH₄-N fertilizer N sources in central Montana is likely a result of preferential microbial assimilation of NH₄-N.