Preface to the special section 'Recent developments on dynamics of substances in the rhizosphere'

The rhizosphere, as defined by Dr. Lorenz Hiltner in 1904, is recognized as the region of soil influenced by plant roots. As the interface of a plant and soil, the rhizosphere is tightly involved in nutrient dynamics. Plant roots have functions not only to uptake water and mineral nutrients but also to release a wide diversity of substances that have strong effects on nutrient availability in soils. Root-secreted compounds have important roles affecting plant–microbial interactions and symbioses, such as nodules and mycorrhiza. Although the term ‘rhizosphere’ was established as important many years ago, many difficulties particularly related to spatial and temporal constraints have hindered approaches to clarify phenomena found in the rhizosphere. Nevertheless, recent progress of analytical methods including omics analyses such as metagenomics, transcriptomics, and metabolomics have supported breakthroughs in rhizosphere research. In fact, publications including ‘rhizosphere’ as a keyword were 6.2 times more numerous in 2020 than in 2000. Recent developments of methodologies such as multiomics analyses using two or more methods of omics approaches, surface analysis, and single-cell analysis have supported great strides in elucidating rhizosphere phenomena. The first international conference on the topic of rhizospheres was held in 2004 in Munich, Germany. The rhizosphere conference has been held every 3–4 years since that first conference, with increasing attendance. Many researchers in broad fields involved in the study of rhizospheres have made important presentations at the recent conference in 2019 at Saskatoon, Canada. Interconnections among related fields including both soil and plant sides have re-emphasized the fields’ importance for further understanding. Recognition of the rhizosphere as an important topic has also been extended by the Japanese Society of Soil Science and Plant Nutrition. Soil Science and Plant Nutrition (SSPN) edited a special section ‘Frontline in the Rhizosphere Research Involved in Phosphorus: for Efficient Use of Unavailable P in Soils (Rhizo-P)’ in 2018. This Special Section, ‘Recent developments on dynamics of substances in the rhizosphere’, has been proposed to introduce current research progress in the fields. White lupin, which is well known as a cluster root forming plant species showing exudative bursts, is addressed in articles on the mineral mobilization topics. Involvement of phytohormones in the unique morphology of roots has been newly suggested in an article. Novel findings related to plant responses to nutrient deficiency and stress environments are also presented in this issue. Topics involved in the symbioses and competition between plants and microbes, and the related substances are also important. The spatial dynamics of mineral nutrients are also addressed. Research papers presented in the section were collected through announcements by the Guest Editorial Board (Dr. Jun Wasaki [chief], Dr. Takuro Shinano, Dr. Naoko Ohkama-Ohtsu, Dr. Junpei Takano, and Dr. Kyoko Miwa). Many manuscripts have been contributed from a wide range of rhizosphere researchers including attendees of the rhizosphere conference. They were peer reviewed entirely using the same processes as other papers published in SSPN. It is my sincere hope that the papers include reports from the front line of research challenges for the advancement of work on important topics and that they contribute to further progress in understanding of the dynamics of substances in the rhizosphere in the future. Last but not least, I thank all the reviewers for their valuable comments and the SSPN Editorial Board for giving us the opportunity to publish this Special Section.