Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters

Abstract

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 alkaliextractable 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.

Effects of extraction conditions on the redox properties of soil organic matter (SOM) and its ability to stimulate microbial iron(III) mineral reduction by electron shuttling

“…However, a previous microbial Fe(III) mineral reduction study of 5 mM ferrihydrite in the presence of 50 mg C L -1 OM, 0.8 mM phosphate buffer and 2×10 5 cells mL -1 Shewanella oneidensis MR-1 showed no goethite or magnetite (based on 57 Fe-Moessbauer and XRD analysis) but vivianite as the major mineral phase (Amstaetter et al, 2012). The transformation of ferrihydrite to vivianite instead of goethite or magnetite in presence of phosphate buffer was also reported in other studies using similar concentration of OM, buffer, cells and ferrihydrite (Chen et al, 2003a;Piepenbrock et al, 2011;Shimizu et al, 2013). The formation of more crystalline secondary mineral phases such as goethite was only observed during ferrihydrite reduction in the absence of phosphate (Hansel et al, 2003;Borch et al, 2007).…”

“…Therefore, they believed that very minor chemical modifications of NOM happened during the chemical isolation procedure (Olk et al, 2019). In contrast, in another study (Li et al, 2016), NOM was also extracted from Suwannee river using a styrene-divinylbenzene copolymer (PPL)-based solid phase extraction (SPE) (PPL-SPE_NOM) and the same RO method as used in Olk et al (2019) (RO-NOM).…”

Biostimulant Action of Dissolved Humic Substances From a Conventionally and an Organically Managed Soil on Nitrate Acquisition in Maize Plants

“…The natural occurrence and role of humic substances in soils was questioned (Lehmann and Kleber, 2015) because of the harsh alkali-based procedures used for their extraction. However, the usefulness of the humic substancesbased approach to understand natural organic matter processes has been recently confirmed (Olk et al, 2019). Furthermore, DHS can be obtained without the use of alkaline extractants by simply leaching soil with water.…”

“…Although OF is claimed to improve organic matter-related soil quality, evidence of the effects of OF on the amount and biological activity of humic substances is still lacking. In addition, while soil use and management have been recognized to have a significant impact on humic substances' complexity and activities (Nardi et al, 2004;Olk et al, 2019), it is not known whether the presence of a crop can affect in itself the quality and quantity of DHS.…”

Long-term persistence of soil organic matter in Amazonian Dark Earth

“…In this 'new' view, the role of soil minerals in organo-mineral associations that protect SOM from being further decomposed by the soil microbial community is highlighted (Baldock and Skjemstad, 2000;Torn et al, 1997). More information on the disputed 'traditional' and 'new' views on SOM is addressed in the discussion between Kleber and Lehmann (2019) and Olk et al (2019). Moreover, whether this 'new' view is indeed a novel approach is addressed by Baveye and Wander (2019).…”