Microbial Immobilization of Nitrogen‐15 Labelled Ammonium and Nitrate in an Agricultural Soil

Fertilizer N recovery is sometimes greater for NO 3 –N than for NH 4 –N forms by no‐till wheat ( Triticum aestivum L.) in the semiarid northern Great Plains and is perhaps affected by preferential N immobilization. This study was conducted to determine whether fertilizer N immobilization (FNI) was affected by N form (NH 4 –N vs. NO 3 –N) and glucose‐C availability. Laboratory incubation (26 d, 10°C) was conducted on a Tamaneen clay loam (fine, smectitic, frigid Typic Agriustolls) collected from a field site where greater recovery of NO 3 –N compared with NH 4 –N fertilizer forms (i.e., NaNO 3 vs. urea) had been previously observed by wheat. Soil (20 g) was treated with 2 mg of N as 15 N‐labeled NaNO 3 and (NH 4 ) 2 SO 4 in factorial combination with a gradient of 0, 10, or 100 mg of glucose‐C. Inorganic N (NH 4 –N + NO 3 –N) concentrations were comparatively stable over the 26‐d incubation without C but exhibited a biphasic time response with C, falling rapidly during the first phase to a lower step second phase. Overall, inorganic N concentrations were lower for (NH 4 ) 2 SO 4 than NaNO 3 . Fertilizer N immobilization vs. incubation‐day relationships conformed to an exponential rise to maximum functions. At 26 d, FNI was equivalent to 0.87, 27.6, and 65.9% for NaNO 3 and 8.8, 41.2, and 65.3% for (NH 4 ) 2 SO 4 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. This study found that NH 4 –N was preferentially immobilized over NO 3 –N where available C was limited and likely contributing to greater fertilizer N recovery of NO 3 –N forms by wheat.