Nitrogen fertilizer management on cotton (Gossypium hirsutum L.) yield and quality in two tropical soils

Urea is the most consumed nitrogen (N) fertilizer but is susceptible to high N losses that can result in lower crop yields compared to other N sources. The aim of this study was to evaluate the effects of the continuous usage of urea and calcium ammonium nitrate (CAN) at different N rates on cotton yield, fiber quality, nitrogen agronomic efficiency (NAE), and soil N stock. Two field trials were conducted across four seasons in two regions of Brazil with contrasting soil textures, with clayey and sandy loam soil. The experiment design consisted of two N sources (urea and CAN), four N rates (48, 96, 144, and 192 kg of N per hectare [kg ha -1]), a control plot (no N), and an additional treatment (different N split). Cotton was sown in the rainy summer season using the cultivars TMG 47 B2RF and DP 1536 B2RF, where rows were spaced 0.90 and 0.76 m apart, with densities of 100,000 and 90,000 plants ha-1, in the clayey and sandy loam soils, respectively. Climate conditions were favorable (>800 mm rainfall and 26oC) during growing, and cotton yield ranged from 2514 to 5437 kg ha -1. Nitrogen application increased cotton yield for both N sources in clayey soil, but not in sandy loam, probably due to N mineralization and a low C/N ratio. In the clayey soil, the N addition at 192 kg ha- 1 increased cotton yield by up to 50 % (3500 to 5300 kg ha- 1 in the 2019 season) and increased the accumulated yield (4 years) from 12,000 to 16,000 kg ha- 1 compared with control. Over the four seasons, CAN application resulted in a higher yield per unit of N compared with urea, resulting in means for NAE of 10.3 and 8.3 kg kg -1, respectively. The N addition improved the micronaire index from the base range, 4.3-4.8 micrograms per inch (control), to premium values of 3.8-4.3 and had no effect on other cotton quality properties or soil N stock. The management of N plays a major role in cotton production and quality, which depend on site-specific conditions. In the long term, yield responsiveness to N was intensified, which suggests that further differences may occur between urea and CAN.