Artificial Root: Development Based on Synthetic Rhizosphere Fertilizers for Agriculture under Environmental Stresses

Abstract

Artificial Root (AR) concept for agriculture is introduced following the integration of interdisciplinary research by the present author and the literature survey of the relevant topics in order to achieve sustainable and efficient plant and crop yield under the emerging environmental stresses due to climate change. The AR system utilises the plant’s own defence mechanisms through the generation of a protective microenvironment for the plant roots, which results in the protection of the rhizosphere and the enhancement of plant exudate concentration. The bioactive forms of AR can also enhance or provide nitrogen fixation by the plant and bacteria in the protected rhizosphere. The emerging stresses include enhanced CO 2 , elevated temperatures, extreme weather, and drought events, which AR can address for soil fertility, CO 2 sequestration, plant response to stressors, and crop productivity. The AR system is administered into the soil as a synthetic rhizosphere (SRS) fertilizer (denoted as SX-fertilizer, X = A, B depending on fertilizer type) and shortly becomes part of the growing plant when the plant roots preferentially ingress into the skeleton of the fertilizer particles through bio-, chemo-, and hydro-tropisms. Once associated with the plant roots, AR functions both as rhizosphere and rhizosheath. At this stage, the skeleton of the SX-fertilizer particles is a microporous, interconnected, elastic, hydrophilic, bioactive, and functionalized material, such as PolyHIPE Polymer (PHP). PolyHIPE is chosen because of its biological, chemical, and morphological versatility, which allows the evaluation of the prevailing mechanisms in AR. The skeleton of the bioactive SB-fertilizers contains suitable bacteria/fungi for root infection and nitrogen fixation in legumes or nitrogen-fixing bacteria for non-legumes. SA-fertilizers without bacteria are produced in situ during the low temperature and pressure ammonia production by plasma catalytic synthesis with reactive separation using PHP. SX-fertilizers not only improve soil fertility, but also enhance plant productivity/yield under water and nutrient stress by 50% - 300%. The enhancement of root function, including root hair and exudate production, and bacterial ingrowth from the soil is dependent on the biochemical and physical structure of PHP. The available literature on the use of PHP in medicine, biotechnology, and tissue engineering is reviewed for the optimal design of the SB-fertilizers’ biochemical and physical structures. SB-fertilizers can replace the use of Haber Bosch ammonia-fertilizers. SX-fertilizers’ use represents negative carbon emission as they can sequester carbon over a long-time scale. Other emerging fertilizers include the use of nano-silica particles as a sub-class of quantum-(QX) fertilizers. Highly porous QX-fertilizers, based on nano-SiO 2 with B, Ca, Mg, Mn, Fe, Co, Ni, Cu, Zn, Mo, V, Se nano-spinel oxides are obtained readily when high/medium entropy supported quantum catalysts, QCs (primarily for use in plasma catalytic reactions), reach their service life, thus providing a circular bio-economy. In the case of nitrogen fixation through catalytic plasma activated water technique using QCs, the resulting NH 4 + NO 3 − solution can be used without catalyst removal.