Crop nitrogen use efficiency for sustainable food security and climate change mitigation

Worldwide food security through the Green Revolution relied heavily on fertilizers, of which nitrogen (N) is the most predominant input in the form of urea, ammonium nitrate, diammonium phosphate, etc. However, poor crop nitrogen use efficiency (NUE) at a global average of about 33% results in avoidable N losses that have negative economic and environmental consequences. N pollution of air and water adversely impacts health, biodiversity, and climate change. Nitrous oxide is a greenhouse gas 300 times more powerful than carbon dioxide and has already replaced methane as the second largest greenhouse gas emitted from Indian agriculture. Agronomic best practices such as legume-based crop rotation, manure and organics management, choosing the right N fertilizer and applying it at the right time in the right quantity, and adoption of precision agriculture can reduce N losses to some extent. However, improving the inherent biological capacity to use the available N efficiently is necessary for crop improvement. NUE is an inherited, multigenic, and quantitative trait spanning several processes including N acquisition, assimilation, translocation, and internal remobilization in plants. Lack of proper characterization of the phenotype and genotype for NUE has delayed crop improvement so far. However, the last decade has witnessed several developments in this field for selection and breeding, including molecular breeding and transgenics. This chapter reviews the current knowledge on these aspects and the emerging prospects for biological improvement of crop NUE toward sustainable and climate-friendly agriculture.