Viewpoints on the Future of Humic Substances Research

Identification of next-generation International Humic Substances Society reference materials for advancing the understanding of the role of natural organic matter in the Anthropocene
by Chin et al., Aquat. Sci. 85, 2023, 32
doi: 10.1007/s00027-022-00923-x (Link to the article)

Many challenges remain before we can fully understand the multifaceted role that natural organic matter (NOM) plays in soil and aquatic systems. These challenges remain despite the considerable progress that has been made in understanding NOM’s properties and reactivity using the latest analytical techniques. For nearly 4 decades, the International Humic Substances Society (IHSS, which is a non-profit scientific society) has distributed standard substances that adhere to strict isolation protocols and reference materials that are collected in bulk and originate from clearly defined sites. These NOM standard and reference samples offer relatively uniform materials for designing experiments and developing new analytical methods. The protocols for isolating NOM, and humic and fulvic acid fractions of NOM utilize well-established preparative scale column chromatography and reverse osmosis methods. These standard and reference NOM samples are used by the international scientific community to study NOM across a range of disciplines from engineered to natural systems, thereby seeding the transfer of knowledge across research fields. Recently, powerful new analytical techniques used to characterize NOM have revealed complexities in its composition that transcend the “microbial” vs. “terrestrial” precursor paradigm. To continue to advance NOM research in the Anthropocene epoch, a workshop was convened to identify potential new sites for NOM samples that would encompass a range of sources and precursor materials and would be relevant for studying NOM’s role in mediating environmental and biogeochemical processes. We anticipate that expanding the portfolio of IHSS reference and standard NOM samples available to the research community will enable this diverse group of scientists and engineers to better understand the role that NOM plays globally under the influence of anthropogenic mediated changes.


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.


Advances in the determination of humification degree in peat since Achard (1786): applications in geochemical and paleoenvironmental studies
by Zaccone et al., Earth-Sci. Rev., 185, 2018, 163-178
doi: 10.1016/j.earscirev.2018.05.017 (Link to the article)

The humification process is one of the least understood and most intriguing aspects of humus chemistry and vital to the global carbon (C) cycle. Peatlands represent the largest terrestrial reservoirs of organic C and support a unique biodiversity, but are also natural archives of climate and environmental changes. In fact, cores from ombrotrophic peatlands are commonly used to reconstruct environmental impacts by human activities during the past decades, centuries and millennia. Understanding the extent to which bogs may serve as reliable archives is of paramount importance in order to ensure that chronological information about natural and human-induced environmental changes are effectively preserved in peat deposits rather than irretrievably affected by humification. Structural changes of the organic matter which constitutes peat are often evaluated by various indirect measures of the degree of humification. Numerous methods and proxies have been proposed and used during the last century, often providing contrasting or inconsistent results. Here, we tested multiple physical, chemical, spectroscopic and thermal approaches using peat samples collected from nine bogs on four continents. Despite the different climatic conditions, botanical composition, depth and age of peat formation, we show that the H/C ratio is the simplest, most widely valid and cost-effective humification proxy and should find universal application to multi-proxy geochemical and paleoenvironmental studies. Moreover, we show that, on average, three-quarters of the organic C in the studied bogs is in a comparatively undecomposed state, and thus vulnerable to climate changes.