Precision Nitrogen Management in Bt Cotton ( Gossypium hirsutum ) Improves Seed Cotton Yield and Nitrogen Use Efficiency, and Reduces Nitrous Oxide Emissions

Field experiments were conducted to establish precision N management technology for nitrogen (N) topdressings in Bt ( Bacillus thuringiensis ) cotton ( Gossypium hirsutum ) along with the quantification of nitrous oxide emissions from the soil. The treatments consisted of five fixed N rates (0, 60, 90, 120 and 150 kg N ha −1 applied in two split doses) and eight site-specific N management (SSNM) treatments of applying different N rates at 40 days after sowing (DAS) and the application of leaf colour chart (LCC)-guided 30, 45 or 60 kg N ha −1 at 55 DAS only in four treatments and at both 55 and 85 DAS (first flowering and boll formation stages) in the remaining four treatments. A higher R 2 value between the total N uptake and the leaf N concentration at 85 DAS than at 55 DAS strongly suggested that fertilizer N management based on leaf N concentration measured in terms of LCC scores led to an adequate total N uptake resulting in a respectable yield. Topdressings of 45 to 60 kg N ha −1 at 40 DAS and an LCC-based application at 55 DAS produced seed cotton yields on a par with the addition of 150 kg N ha −1 based on a soil test basis along with an improved recovery and agronomic efficiency use, and remarkably lowered the nitrous oxide emissions as estimated using the CCAFS (Climate Change, Agriculture and Food Security)-Mitigation Option Tool.