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 R2 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.
Highlights
An inadequate supply of nitrogen (N) to Bt (Bacillus thuringiensis) transgenic cotton hybrids impacts their yield and protein synthesis and metabolism, which is critical in transgenic crops as it can potentially alter the cry toxin expression [1]
The nitrogen concentration increased with an increasing leaf colour chart (LCC) score as recorded from the fourth leaf from the top at both growth stages (55 and 85 days after sowing (DAS)) of Bt cotton
The R values of the LCC score and leaf N concentration were greater at 85 DAS than at 55 DAS, indicating that the R2 values of the relationships between the LCC score and leaf N concentration gave a good indication of the leaf N status
Summary
An inadequate supply of nitrogen (N) to Bt (Bacillus thuringiensis) transgenic cotton hybrids impacts their yield and protein synthesis and metabolism, which is critical in transgenic crops as it can potentially alter the cry toxin expression [1]. N recommendation leads to an N application in excess of the needs of the crop in many cases, resulting in low fertilizer N use efficiency and enhanced nitrous oxide emissions from the soil to the environment [2]. Cotton growers mostly do not take the risk of low yields due to limiting N and apply subsidized fertilizer N above the standard recommendation. Little attention is paid to low N use efficiency and the damage that can be done to the environment or soil health. In north-western India, the standard fertilizer N recommendation for Bt cotton consists of broadcasting a fertilizer N dose of 75 kg ha−1 at about 40 days after sowing (DAS) when at each hill only one plant is retained and another 75 kg N at 55 DAS i.e., the first flowering stage [3]. Need-based N management strategies help to synchronize the fertilizer N supply and crop demand and thereby maximize the N uptake
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