European Journal of Sustainable Agroecosystems

Overview: The three cereal crops of wheat (Triticum aestivum L.), maize (Zea mays L.), and rice (Oryza sativa L.) form the backbone of food security at a global level. However, global productivity of these crops is limited by their intrinsic low efficiency of nutrient use due to the traditional bulk fertilizers which allow recovery of only 20-50% of nitrogen and 20-30% of phosphorus applied to the soils. The balance is lost through various processes such as leaching, volatilization, fixation, and surface runoff. Nano-fertilizers (NFs) are a concept coming from the field of precision agriculture and consist of nutrients formulated in the 1-100 nm range for the purpose of controlling the flux of nutrients in rhizosphere and enhancing the ions acquired by the roots of the crops through controlled process of dissolution, higher specific area, and nano-bio interface reactivity. 
Scope: This review presents a critical analysis of peer-reviewed studies on the functioning of nano-fertilizers in terms of modulation of rhizospheric bioavailability of nutrients and transport of ions through the root system in cereal crops grown under nutrient-limiting conditions. 
Data sources: A systematic literature search was conducted using Scopus, Web of Science, PubMed/PMC, ScienceDirect, SpringerLink, Wiley and Frontiers for the key findings include that nano-fertilizers improve nutrient use efficiency (NUE) by over 50%, and increase the yields of cereals by 13-55% compared to conventional fertilizers through low application rates. This is achieved mainly through diffusion-limited release of ions, low molecular weight organic acid (LMWOA) induced dissolution of nanoparticles in the rhizosphere, stimulation of plant growth promoting rhizobacteria, and regulation of expression of both high and low-affinity transporters (NRT1/NPF, NRT2, AMT, PHT1, HAK/AKT). CBL-CIPK23 signaling module is established as a key participant in the coordination of co-transport of nitrate and potassium, which is mediated through nano-created ionic microenvironments.
There are several research gaps identified in the study, such as the absence of mechanisms which connect the dynamics of transformation of nanoparticles to the control of the gene expression level of transporters in cereals, and the non-linearity of the dose-responses of nano-sized fertilizers as well as dependence on the type of soil, and lack of investigation of long-term ecotoxicology in the rhizosphere and the issues of field experiments.