Soil–Plant–Microbe Interactions: The Role of Biostimulants and Agro-Environmental Implications

Dear Colleagues,

This Special Issue, edited by Dr. Regina Gabilondo and Dr. Zhang, will continue with the topic initiated by the preceding Special Issue on the "Effect of Biostimulants in the Soil–Plant–Microbe System" and will delve into the mechanisms of action of biostimulants and the interactions and processes of the soil–plant–microbe system. Regulation (EU) 2019/1009 defines the term biostimulant as “an EU fertilizer product whose function is to stimulate the nutritional processes of plants regardless of the nutrient content of the product, with the sole objective of improving one or more of the following plant characteristics and its rhizosphere: efficiency in the use of nutrients, tolerance to abiotic stress, quality characteristics or availability of nutrients immobilized in the soil and the rhizosphere”. There are different types of biostimulants, including those based on beneficial microorganisms (bacteria and fungi), algae, and chitosan products. Those belonging to the group called “plant growth-promoting rhizobacteria” (PGPR) are among the most commonly used treatments in plant cultures. Most of the beneficial organisms used as biostimulants for plants live in the rhizosphere, a region around the plant roots with intense microbial and faunal activity, where the plant nutrient absorption, a process that is still not fully understood, takes place. Plants secrete large amounts of molecules, called exudates, into the soil through their roots and which stimulate microbial activity in the rhizosphere. These exudates provide carbon to the soil microorganisms, mainly fast-growing bacteria, that are strongly carbon-limited and are thereby increased together with microfaunal grazers such as bacterial-feeding protozoa and free-living nematodes. The exudate carbon comes from the total net fixed carbon by plants, representing some 10–20% of it. Another 10–20% is consumed by microbial symbionts, such as mycorrhizae or N₂-fixing microorganisms. Supporting microbial interactions in the rhizosphere is of great importance for plants because the availability of mineral nutrients to plants is strongly enhanced via the microbial loop. Bacteria feed on the soil nutrients in different ways, liberating them through microfaunal grazing and increasing their bioavailability for plants. Biostimulants provide beneficial microorganisms, nutrients, vitamins, and other compounds that interact with the soil and the rhizosphere of plants, with the aim of promoting their growth and yield.

In this Special Issue, research advances will be presented to aid understanding of the role of biostimulants, soil–plant–microbe interactions, ecology and biochemistry, biostimulant mechanisms of action, and agro-environmental implications.

Dr. Regina Gabilondo
Prof. Dr. Ling Zhang
Guest Editors

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• soil–plant–microbe system
• biostimulants
• rhizosphere
• fertilizers
• availability of mineral nutrients
• microfaunal grazing
• protozoa
• nematode
• mycorrhizae
• microbial loop