Ammonia volatilization from NBPT-treated urea in no-till maize at different soil textures in South Brazil

Abstract

Urea treated with urease inhibitors can significantly reduce NH₃ losses when urea is surface applied to the straw in a no-till (NT) system, but the site conditions may affect its efficiency. The aims of this study were to evaluate the efficiency of the NBPT [N-(n-butyl) thiophosphoric triamide] applied to common urea on the decrease of NH₃ volatilization in no-till maize in two sites with distinct soil textures, weather, and management in southern Brazil; and to evaluate the NBPT inhibition in delayed soil pH and NH₄⁺ elevation after urea hydrolysis. Two field experiments were conducted in the 2016/17 growing season at Guarapuava (GUA) (Rodhic Hapludox, clay) and Eldorado do Sul (ELD) (Typic Paleudult, sandy clay loam). The treatments were: (i) common urea (U) and (ii) U + urease inhibitor (U + NBPT), and (iii) control without N fertilization. The N sources were broadcast applied to maize at N rate of 100–180 kg ha⁻¹. In the laboratory, an experiment was conducted using U and U + NBPT, applied to the two soils, to evaluate urea hydrolysis. The NH₃ volatilization from U was faster and higher in the sandy soil compared to clayey soil, reaching 26 % of N applied in the first five days after application, compared to 10 % in GUA. In this period, the NBPT reduced the NH₃ loss from urea by 83 % in sandy soil (ELD) and 88 % in GUA (clay). After this period, rain events occurred, which favoured the diffusion of N from fertilizer in ELD, but not in GUA, where NH₃ losses increased from 10 to 14 % with U, and from 1 to 7 % with U + NBPT, reducing the efficiency of the inhibitor. Under controlled conditions, U application increased soil NH₄⁺ levels up to 207 to 224 mg kg⁻¹, and soil pH up to 6.8–8.1 in five days after application, while in U + NBPT the NH₄⁺ peak was reduced to 72 to 109 mg kg⁻¹ and pH to 6.2–7.2, showing similar efficiency in both soils. Nitrogen losses through NH₃ volatilization were intense during the first five days after urea application in no-till maize, especially in sandy soil. NBPT-treated urea is an effective N management practice in reducing ammonia losses in both sandy and clay soils; however, its efficiency decreases over time in the clay soil besides rainfall events. Further studies should investigate additional strategies to optimize NBPT efficiency in reducing NH₃ loss from urea in no-till systems in clay soils.