Evaluation of Nitrification Inhibitors and Urea Placement in No-Tillage Winter Wheat

Nitrification inhibitors (NIs) such as dicyandiamide (DCD) and nitrapyrin [2-chloro-6-(trichloromethyl)pyridine] (NP) slow the nitrification process, thus reducing N losses from leaching and denitrification. In field studies (1990–1994) on a Renfrow silt loam (fine-silty, mixed, thermic Udertic Paleustolls) near El Reno, OK, we evaluated these two NIs and their placement effects on nitrification of applied urea in soils and on wheat (Triticum aestivum L.) yields. Urea at 60 kg N ha⁻¹ treated with or without NIs was placed 3 to 4 cm below the seed (BL) at seeding or was broadcast (BR) after seeding on day of seeding, with no N as control. Mineral N forms were determined in the 0- to 30-cm soil zone in fall and spring. Differences in soil mineral N concentration below the 10-cm depth were minimal (but higher than the control) in all N treatments. In the 0- to 10-cm depth, BL placement of NI-treated urea significantly reduced nitrification in the fall compared with BR urea with or without NIs, except in 1994. Soil mineral N content in spring was similar with either placement of NI-treated urea, but was significantly greater than untreated BR urea. Warm temperatures and dry soil in the fall tended to reduce the effectiveness of surface-applied NIs. The highest NH₄⁺ percentage in total soil mineral N was observed with DCD BL or NP BL, followed by NP BR and DCD BR. Urea treated with NIs increased grain yield from 7 to 31% above untreated urea. Grain yield was highest with DCD BL (31%), followed by DCD BR (26%) and NP BR (24%), compared with the broadcast untreated urea. Grain N yield was 140 g kg⁻¹ greater with DCD-N than with untreated urea. Deep placement of DCD under no-till winter wheat in the southern Great Plains can reduce nitrification and increase N availability under varying year-to-year environmental conditions and can improve wheat grain yield and grain N compared with surface broadcast of urea with or without NI.