Role of Humification Processes in Recycling Organic Wastes of Various Nature and Sources as Soil Amendments

Recycling the large amounts of organic wastes produced by agriculture, forestry, urban and industrial activities as soil, organic amendments are the most popular and efficient option for avoiding their dispersion in the environment and restoring, maintaining, and/or improving the content of soil organic matter. Chemical stability and biological maturity are two important factors for the successful use of organic wastes in agriculture with limited risk for the surrounding environment. Stabilization and maturation of raw organic wastes inherently imply the achievement of an extensive humification, that is, a wide conversion of easily degradable organic matter to refractory organic compounds that resemble native soil humic substances (HS). Soil HS are the most important components of soil organic matter responsible of several soil functions and processes. As a consequence, the amount and quality of HS‐like fractions in any organic amendment are believed to be of primary importance for its agronomic efficacy, environmental safety and economic value. The first part of this review focuses on the chemical and physico‐chemical changes occurring in the humic substances (HS) ‐like fractions of organic wastes of various nature and sources subjected to common treatment processes aimed at producing environmentally‐safe soil amendments with beneficial agronomic properties. The second part discusses the composition, structure, and chemical reactivity of the HS‐like components in organic amendments of various origins and nature, and their effects on native soil HS. The review concludes by highlighting the need for innovative research targeted mainly to achieve a better fundamental understanding of the molecular structure and reactivity of soil HS and HS‐like fractions in organic amendments, the mechanisms of HS formation and transformations in the natural environment and during the treatment processes of raw organic wastes, the interactions with metals and organic xenobiotics, and the direct physiological effects that HS may exert on plants.