Crop nitrogen (N) utilization mechanism and strategies to improve N use efficiency

Nitrogen (N) is an essential plant macro-nutrient for crop sustainability and productivity. Substantial quantities of N fertilizers are being applied in soil, but only about 33% is utilized by the plants. Its availability in soil varies to a great extent in terms of time and space. Plant root systems should efficiently respond to fluctuating N by tailoring root growth and development. However, N fertilizer production consumes massive energy resource, and excessive application has negative consequences on the environment and human health. Therefore, innovative solutions are imperative to enhance crop yields and N use efficiency (NUE) simultaneously, while maintaining and/or reducing N application amount. Crop NUE is a complex attribute, because it is controlled by numerous genetic as well as environmental factors interact to govern the mechanisms involved in N sensing, uptake, translocation, assimilation, and remobilization in plants. Hence, a better understanding of these mechanisms is a key factor for improving NUE in cropping systems. In this review, we discussed the molecular, biochemical, and enzymatic mechanisms involved in NUE in crop plants, ways to increase NUE through the identification of plant factors with special consideration of their interaction, and different management strategies. In addition, adaptation of classical approaches, i.e., root architecture studies, quantitative trait loci (QTLs), and selection of genes for better NUE, are briefly discussed. Broadly, from root uptake to accumulation of N assimilates in various plant tissues, an array of physiological mechanisms is involved which is still not fully understood. Moreover, employing an integrated approach by combining expertise from fundamental and applied investigations in crop sciences may add further to available knowledge regarding crop N utilization.