Nitrogen-Fixing, Phosphorus-Solubilizing, and Potassium-Solubilizing Microorganisms: Key Players in Plant Nutrition

Dear Colleagues,

Soil is a complex system of minerals, water, air, organic matter, and billions of organisms, with several changes and interactions with plants taking place over time. Soil fertility refers to the capacity of soil to supply essential nutrients to plants, such as N, P, K, and micronutrients, which are usually not available in a free form or are only found in limited quantities in the soil. Nitrogen fixation is an important process for increasing plant productivity in both natural and agricultural ecosystems. To increase nitrogen availability to plants, it is a common practice to apply mineral or organic fertilizers to arable soil. However, it is important to reduce the adverse impacts of nitrogen pollution, support plant growth, improve soil fertility, and retain soil moisture, with practices such as the use of green manure, which is beneficial in weed management, fostering food security and sustainable agriculture. Biological nitrogen fixation (BNF) is a sustainable alternative to synthetic N₂ fertilizers. Diazotrophic microorganisms, such as Azotobacter, Azospirillum, and Rhizobium, and N₂-fixing cyanobacteria can convert atmospheric N₂ into bioavailable ammonia for plant uptake, thus increasing N₂ availability and promoting the nutrient use efficiency (NUE) of crops. Nitrogen fixation is performed by different types of bacteria and archaea using the enzyme nitrogenase. BNF is considered an extremely sensitive process influenced by environmental and nutrient conditions and enables plants to obtain all or some of their required nutrients through interactions with associative, endo-symbiotic, and endophytic symbionts. Phosphorus is another important nutrient for plant growth and development, playing a vital role in plant physiology and reproduction. Phosphate-solubilizing microorganisms (PSM) are a group of organisms composed of actinobacteria, arbuscular mycorrhizae fungi, and cyanobacteria capable of hydrolyzing inorganic and organic phosphorus into soluble forms, thus making it bioavailable to plants. Phosphorus is present in fundamental molecules of life, such as phospholipids, nucleic acids, adenosine triphosphate (ATP), and other indispensable biological substances. Potassium is one of the most important nutrients for plants. It is essential for protein and starch synthesis, water and nutrient transport under salinity stress, the activation of enzymes, osmoregulation, and stomatal closure and opening. Potassium is one of the nutrients most significantly impacted during salt stress, which can negatively influence plant root growth. Potassium-solubilizing rhizobacteria (KSR) show considerable potential in agriculture as eco-friendly, nature-based solutions for mitigating the rhizospheric potassium dilemma. These bacteria use biochemical processes such as acidolysis, complexation, and chelation to convert fixed potassium into plant-available forms. It is known that potassium-solubilizing bacteria (KSB) can solubilize K-bearing minerals and convert the insoluble K into soluble forms of K available for plant uptake. The interactions between microorganisms and plant roots have direct effects on the soil and the rhizosphere region, which is the home of a diverse variety of microorganisms, such as archaea, protists, fungi, bacteria, and viruses which reside inside or on their host plants. The microbial population can also promote plant development and respond to a wide range of environmental conditions.

In this Special Issue, we highlight the mechanisms and importance of nitrogen-fixing, phosphorus-solubilizing, and potassium-solubilizing microorganisms in relation to plant nutrition. Moreover, this Special Issue aims to study the molecular mechanisms underlying the contents of root exudates under nutrient deficiency, studying different microbes as well as their influences on alleviating nutrient scarcity effectively.

Dr. Wenli Sun
Dr. Mohamad Hesam Shahrajabian
Dr. Spyridon A. Petropoulos
Guest Editors

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• arbuscular mycorrhizal fungi
• Asteraceae
• mineral nutrition
• nematodes
• nitrogen deposition
• phosphorus-solubilizing
• plant-growth-promoting rhizobacteria
• potassium-solubilizing microorganisms, rhizobacteria
• root exudates
• secondary metabolites
• soil bacterial community
• soil microorganisms