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

Abstract

Fertilizer N recovery is sometimes greater for NO3–N than for NH4–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 (NH4–N vs. NO3–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 NO3–N compared with NH4–N fertilizer forms (i.e., NaNO3 vs. urea) had been previously observed by wheat. Soil (20 g) was treated with 2 mg of N as 15N-labeled NaNO3 and (NH4)2SO4 in factorial combination with a gradient of 0, 10, or 100 mg of glucose-C. Inorganic N (NH4–N + NO3–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 (NH4)2SO4 than NaNO3. 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 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/FNINaNO3, averaged (1–26 d) 10.7, 1.8, and 1.5 for 0, 10, and 100 mg glucose-C, respectively. This study found that NH4–N was preferentially immobilized over NO3–N where available C was limited and likely contributing to greater fertilizer N recovery of NO3–N forms by wheat.