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Fertilizer nitrogen (N) is the major input for food production. Excessive and untimely fertilizer N topdressings to achieve high yields are common among the farmers in the major food producing regions of Asia. A four year field study was conducted to validate precision N management strategies in wheat (Triticum aestivum L.; variety PbW 550) through optical sensing with leaf colour chart (LCC), chlorophyll meter (SPAD), and GreenSeeker (GS) optical sensor for achieving high N use efficiencies. The study consisted of five treatments--no-N control, soil test based N application, and precision N application using LCC, SPAD meter and GS optical sensor. The precision N management techniques sustained grain yield, N assimilation and root dry weight equivalent to the soil test based fertilizer N application with 16.7 to 25.8 percent less use of fertilizer N during different years. The optical sensing techniques improved agronomic and recovery efficiency of applied N up to 40.5 and 35.3 per cent, respectively in comparison with the soil test based fertilizer N recommendation. The optical sensing based N topdressings promisingly synchronized fertilizer N supply with the plant N demand and are thus more useful in improving N use efficiency, reducing cost of production and mitigating escape of reactive N from soil system to atmosphere.

Nitrogen is one of the most important macronutrients for crop growth and metabolism. To identify marker-trait associations for complex nitrogen use efficiency (NUE)-related agronomic traits, field experiments were conducted on nested synthetic wheat introgression libraries at three nitrogen input levels across two seasons. The introgression libraries were genotyped using the 35K Axiom® Wheat Breeder's Array and genetic diversity and population structure were examined. Significant phenotypic variation was observed across genotypes, treatments, and their interactions across seasons for all the 22 traits measured. Significant positive correlations were observed among grain yield and yield-attributing traits and root traits. Across seasons, a total of 233 marker-trait associations (MTAs) associated with fifteen traits of interest at different levels of nitrogen (N0, N60, and N120) were detected using 9,474 genome-wide single nucleotide polymorphism (SNP) markers. Of these, 45 MTAs for 10 traits in the N0 treatment, 100 MTAs for 11 traits in the N60 treatment, and 88 MTAs for 11 traits in the N120 treatment were detected. We identified putative candidate genes underlying the significant MTAs which were associated directly or indirectly with various biological processes, cellular component organization, and molecular functions involving improved plant growth and grain yield. In addition, the top 10 lines based on N response and grain yield across seasons and treatments were identified. The identification and introgression of superior alleles/donors improving the NUE while maintaining grain yield may open new avenues in designing next generation nitrogen-efficient high-yielding wheat varieties.

In the last six decades, the consumption of reactive nitrogen (Nr) in the form of fertilizer in India has been growing rapidly, whilst the nitrogen use efficiency (NUE) of cropping systems has been decreasing. These trends have led to increasing environmental losses of Nr, threatening the quality of air, soils, and fresh waters, and thereby endangering climate-stability, ecosystems, and human-health. Since it has been suggested that the fertilizer consumption of India may double by 2050, there is an urgent need for scientific research to support better nitrogen management in Indian agriculture. In order to share knowledge and to develop a joint vision, experts from the UK and India came together for a conference and workshop on “Challenges and Opportunities for Agricultural Nitrogen Science in India.” The meeting concluded with three core messages: (1) Soil stewardship is essential and legumes need to be planted in rotation with cereals to increase nitrogen fixation in areas of limited Nr availability. Synthetic symbioses and plastidic nitrogen fixation are possibly disruptive technologies, but their potential and implications must be considered. (2) Genetic diversity of crops and new technologies need to be shared and exploited to reduce N losses and support productive, sustainable agriculture livelihoods. (3) The use of leaf color sensing shows great potential to reduce nitrogen fertilizer use (by 10–15%). This, together with the usage of urease inhibitors in neem-coated urea, and better management of manure, urine, and crop residues, could result in a 20–25% improvement in NUE of India by 2030.

Increasing nitrogen fertilizer applications have sustained a growing world population in the 20th century. However, to avoid any further associated environmental damage, new sustainable agronomic practices together with new cultivars must be developed. To date the concept of nitrogen use efficiency (NUE) has been useful in quantifying the processes of nitrogen uptake and utilization, but we propose a shift in focus to consider nitrogen responsiveness as a more appropriate trait to select varieties with lower nitrogen requirements. We provide a roadmap to integrate the regulation of nitrogen uptake and assimilation into varietal selection and crop breeding programs. The overall goal is to reduce nitrogen inputs by farmers growing crops in contrasting cropping systems around the world, while sustaining yields and reducing greenhouse gas (GHG) emissions.

In the Indo-Gangetic plain in South Asia, fertilizer nitrogen (N) application to irrigated wheat in two split doses at planting and at crown root initiation (CRI) stages of the crop is the general recommendation. Farmers have a tendency to apply another dose of N at maximum tillering (MT) stage to avoid the risk of N deficiency. However, appropriate criteria to decide MT stage application of N are lacking. Field experiments were conducted for three seasons with four wheat cultivars to develop criteria for using a chlorophyll (SPAD) meter and an optical sensor (GreenSeekerTM) for guiding MT stage N application. Application of 0, 60, 80, 100 and 120 kg N ha−1 in two equal split doses at planting and at CRI stage constituted the main plots. The two subplots consisted of applying 0 and 30 kg N ha−1 at MT stage immediately after taking SPAD meter and GreenSeeker readings. Greenness of wheat leaves at MT stage was found to be a function of N applied at planting and CRI stages. Wheat grain yield at maturity was determined by the level of greenness of leaves at MT stage—whether measured by SPAD meter or GreenSeeker (expressed as in-season estimate of yield (INSEY) computed by dividing normalized difference vegetation index by number of days between planting and sensing). Grain yield response to N application at MT regressed against SPAD readings at MT revealed that an application of 30 kg N ha−1 will increase wheat yield by 1.0 or 0.5 t ha−1 when color of the wheat leaves was equivalent to or less than SPAD value of 32.5 or 42.5, respectively. Robust regression between grain yield response and INSEY values revealed that application of 30 kg N ha−1 at MT will lead to a grain yield increase by 1.0 or 0.5 t ha−1 if INSEY values were found to be 0.005 or 0.011, respectively. It should serve as an easy and useful criterion for applying a supplementary dose of fertilizer N at MT stage of wheat.

Blanket fertilizer nitrogen (N) recommendations for large irrigated wheat tracts lead to low N-use efficiency due to field-to-field variability in soil N supply and seasonal variability in yield. To achieve high N use efficiency, a site-specific N management strategy using GreenSeeker™ optical sensor was evaluated. We conducted seven field experiments during 2004–2006 at three locations to define relationships between in-season sensor measurements and yield of wheat and to know whether response of wheat to fertilizer N can be estimated by sensor measurements. During 2005–2007, four field experiments were conducted to assess the sensor-based N management strategy and to work out prescriptive N management to be followed prior to applying sensor-guided fertilizer dose. We observed robust relationships between in-season sensor-based estimates of yield at Feekes 5–6 and 7–8 stages and actual wheat yields. Response of wheat to fertilizer N defined by the sensor was highly correlated with harvest response index. Sensor-guided fertilizer N applications resulted in high yield levels and high N-use efficiency. Application of 90 kg N ha-1 at planting or in two equal doses at planting and crown root initiation stage was the appropriate prescriptive fertilizer N management. This study reveals that high N-use efficiency in irrigated wheat can be achieved by replacing blanket fertilizer recommendation by an optical sensor-based N management strategy consisting of applying moderate amount of fertilizer N at planting and crown root initiation stages and sensor-guided fertilizer N dose at Feekes 5–6 or 7–8 stages of wheat.