Preface - special issue in memory of Frank J. Stevenson

Natural organic matter (NOM) comprises a complex mixture of thousands of organic compounds found in water, soils, and sediments that was naturally formed from residues of plants, microorganisms, and animal matter at various stages of the decaying process. It is ubiquitous and plays an important role for the functioning of ecosystems. In soils it appears as soil organic matter (SOM) and in surface water mostly as dissolved (DOM) or particulate organic matter (POM). However, it occurs also in fossil and recent sediments as well as in composts, sewage sludge, or organic waste. Its chemical composition and physical properties can vary largely and depend on the quality of the substrate, the environmental conditions in which it was formed and the time and intensity of its humification. According to the International Humic Substance Society (IHSS), humification describes the chemical and biochemical processes included in the degradation and transformation of plant and microbial residues into the so called humic substances (HS) (IHSS 2017). NOM is involved in many processes in soils and natural waters, e.g., weathering, plant nutrition, pH buffering, major and trace metal mobility and toxicity, bioavailability, transport of organic chemicals and inorganic compounds, formation of disinfection by-products during water treatment, and heterotrophic production in black-water ecosystems. Bearing this in mind, a better understanding of the character and function of NOM and HS are of broad interest and have received attention from scientists in a wide variety of discipline. Frank J. Stevenson (1922–2015) was one of the scientists who dedicated their professional life to increase our understanding of the properties and functions of NOM. He can be certainly considered as a pioneer of NOM and HS research by exploring innovative approaches to the study of both bulk SOM and humus chemistry. For more than thirty years, he was among the most influential in pushing forward the frontiers and guiding the directions in these research areas, which encouraged the IHSS to dedicate this Special Issue to his memory.