Abstract
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.