Vindication of humic substances as a key component of organic matter in soil and water
by Hayes and Swift, Adv. Agron. 163, 2020, 1-37
doi: 10.1016/bs.agron.2020.05.001 (Link to the article)
Soil organic matter is a complex mixture partly of recognizable, largely unaltered plant components plus a group of highly modified materials that bear no morphological resemblance to the original components. Their formation results from transformation and decomposition processes collectively known as “humification.” Humic substances, a family of closely related compounds, are considered to be a major product of this process. In recent years, several articles have questioned the role and even existence of soil humic substances as a distinct entity in soil organic matter. They regard soil organic matter as a continuum of degradation reactions by unspecified processes from the original biomass inputs to carboxylic acids, and eventually to carbon dioxide without the formation of new classes of compounds along the way. We disagree fundamentally with these views and highlight the errors, misconceptions, misinterpretations, inbuilt bias, lack of scientific rigor, and failure to consider works and evidence that run counter to their preconceived views. In response, we present an evidence-based rebuttal, detailing the decomposition processes of plant components and their transformation to a range of products, some of which, including humic substances, have a degree of resistance to microbial degradation. We provide an appraisal of information about the genesis and compositions of humic substances, and the importance of compositional knowledge to devise treatments to modify their rate of decomposition. We note that all reputable turnover models that accurately predict soil organic matter behavior include one or more persistent pools of carbon which is not compatible with the concepts in the soil continuum model.
The spontaneous secondary synthesis of soil organic matter components: A critical examination of the soil continuum model theory
by De Nobili et al., App. Soil Ecol. 154, 2020, 103655
doi: 10.1016/j.apsoil.2020.103655 (Link to the article)
The “Soil Continuum Model” questions the occurrence of any independent natural process of secondary synthesis that generates compounds structurally distinct from plant or microbial metabolites. This review shows that a vast volume of interdisciplinary scientific evidence supports the formation of relevant non-pre-existing complex molecules exhibiting various types of structures. These molecules form during degradation and decay of biological cell components. The spontaneous abiotic and enzymatically catalysed reactions of components of organic residues and of their oxidative decomposition products suggested by state-of-the-art studies are indeed those proposed by most of the classical descriptions of humification. The review also highlights the chemically active role of pedofauna, explaining why the apparently harsh conditions of alkaline extraction of HS cannot be considered un-natural. Many insects and larvae feeding on foliage of plants with a high content of tannins have a midgut pH above 9. Albeit, reducing conditions are often maintained to avoid oxidation, peroxidases are active in the intestinal tract and pass on to feces. Polyphenols are then immediately enzymatically oxidized to their reactive quinone form, once feces are excreted and exposed to oxygen. Implications of our current knowledge on the reactivity of plant components in soil are discussed in relation with the present state of the art research on humic substances. Contrary to claims by the “Soil Continuum Model” theory, complimentary modern approaches need to be used to understand the complexity of soil organic matter.
Using Humic Fractions to Understand Natural Organic Matter Processes in Soil and Water: Selected Studies and Applications
by Olk et al., JEQ 48, 2019, 1633-1643
doi: 10.2134/jeq2019.03.0100 (Link to the article)
Natural organic matter (NOM) plays key environmental roles in both aquatic and soil systems. A long-standing approach for evaluating NOM composition and activity is to extract soils with alkali solutions to obtain humic substances, namely humic acids (HA), and fulvic acids (FA), or to briefly expose isolated fractions of dissolved organic matter to alkali. Critics have claimed these methods create laboratory artifacts and are thus unsuitable for studying NOM behavior in field conditions. In response, we describe case studies in which humic fractions were analyzed to identify significant processes in environmental or agricultural issues. Specifically, humic fractions played a key role in maintaining toxic levels of arsenic (As) in drinking water supplies in South and Southeast Asia. Elsewhere, binding reactions of FA and HA with prions were shown to provide a plausible mechanism for variable persistence of prion infectivity across soil types. Humic substances were also shown to enhance iron (Fe) uptake by plants in solution culture and field conditions. Their specific binding sites for mercury (Hg) as determined in laboratory conditions enabled accurate modeling of soil Hg binding under varying conditions. A young HA fraction reproduced in controlled conditions the capacity of animal manure to maintain potassium (K) availability in strongly K-fixing field soils, leading to development of a commercially successful humic-K fertilizer. Humic fractions accurately represented NOM across multiple settings and research objectives while providing novel opportunities for advanced analyses. The study of humic fractions has helped resolve scientific and practical issues in aquatic and soil systems.
Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters
by Olk et al., JEQ 48, 2019, 217-232
doi: 10.2134/jeq2019.02.0041 (Link to the article)
To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice (Oryza sativa L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.