From humic substances to soil organic matter–microbial contributions. In honour of Konrad Haider and James P. Martin for their outstanding research contribution to soil science

Abstract

Purpose Many decades of research have shown that humic compounds as part of the soil organic matter (SOM) are essential for the stability and ecosystem services of soils. James P. Martin and Konrad Haider based on several pioneers in humus research improved the basis for the current knowledge of key processes in the soil environment, in particular structure and formation of humus triggered mainly by soil fungi. Other major contributions are briefly described but not in the focus of this article, such as their innovative tracer experiments with isotopelabelled xenobiotic chemicals and natural litter to study their fate. Both scientists inspired generations of younger researchers to advance their approaches and laid the cornerstone of the current understanding of soil organic matter formation. Results and discussion This article values the key experiments of Martin and Haider in this field and the related follow-up research finally resulting in the current view on formation mechanisms of SOM and non-extractable residues (NER) of xenobiotics. The improved understanding of these processes considering necromass with tissue residues of plants, microbes and animals challenges the traditional views of humic matter as macromolecular organic matrix, which according to the research of the last years represents only a variable part of the total organic matter besides associates of low molecular weight molecules. We discuss views on soil organic matter and humic substances that are nowadays considered not to differ in molecular diversity. We will start by demonstrating the understanding of humus characteristics and humus formation three decades ago closely related to the findings of Martin and Haider. Methodological approaches to characterize relevant structural and mechanistic pictures of SOM such as the priming effect, clay mineral catalysed reactions and the various mechanisms by which natural and xenobiotic chemicals are protected in soil are briefly illustrated by examples. Fungal activities in producing secondary metabolites like polyketides and their probably minor contributions to SOM formation are presented. Conclusions and perspectives Open research questions stimulated by these two soil scientists are sketched which are nowadays possible to address by new sophisticated highresolution techniques.

Quantitative assessment of microbial necromass contribution to soil organic matter

“…This apparent conundrum has led to a plethora of relevant research that provided pieces of an emergent understanding centered on microorganisms as actors of SOC stabilization, not only as actors of its mineralization to CO 2 . For example, calls have been made for the explicit consideration of direct incorporation of microbial remnants into slow-cycling soil carbon pools (Gougoulias, Clark, & Shaw, 2014;Liang et al, 2017;Miltner et al, 2012;Schaeffer, Nannipieri, Kästner, Schmidt, & Botterweck, 2015;Schimel & Schaeffer, 2012;Simpson, Simpson, Smith, & Kelleher, 2007), intriguing conceptual frameworks have been developed for organic carbon cycling in soil (Cotrufo et al, 2013;Fan & Liang, 2015;Liang, Cheng, Wixon, & Balser, 2011;Sokol, Sanderman, & Bradford, 2019;Wieder, Grandy, Kallenbach, & Bonan, 2014), and necessary supporting research and databases on microbial necromass dynamics are emerging (Bradford, Keiser, Davies, Mersmann, & Strickland, 2013;Ding, Liang, Zhang, Yuan, & Han, 2015;Kallenbach, Frey, & Grandy, 2016;Kallenbach, Grandy, Frey, & Diefendorf, 2015;Kindler, Miltner, Richnow, & Kastner, 2006;Kindler, Miltner, Thullner, Richnow, & Kastner, 2009;Liang et al, 2016;Ludwig et al, 2015;Ma et al, 2018;Miltner, Kindler, Knicker, Richnow, & Kastner, 2009;Schweigert, Herrmann, Miltner, Fester, & Kästner, 2015).…”

Stabilization of Lead and Copper in a Contaminated Typic Torripsament Soil Using Humic Substances

“…The applied rates of 1.0 and 2.0 g KH kg −1 soil increased the SOM by 23.6% and 27.8% compared with control treatment, respectively; however, soils amended with 0.50 g KH kg −1 showed insignificant increase of SOM. The increased SOM content could be attributed to the organic portion of KH compound, which is about 70–80% of the organic fraction in soil …”

Impacts of vegetation type and climatic zone on neutral sugar distribution in natural forest soils

“…During this process, soil organic matter is formed not only by the degradation of plant-derived organic materials to small organic molecules, but also by the formation of new microbial compounds as C is assimilated into microbial biomass, released upon microbial death, and enters the soil organic matter pool. The awareness of a much higher microbial necromass contribution to stable SOC has been stimulated by empirical data and modeling simulation Miltner et al, 2012;Schweigert et al, 2015;Simpson et al, 2007), but research on this aspect of soil organic matter formation is still lacking Schaeffer et al, 2015).…”

“…Ultimately, SOC is derived from plant primary production (Kögel-Knabner, 2002;Schlesinger and Bernhardt, 2013), but the processes governing the transformation of plant materials into SOC are still being elucidated. There is growing evidence that microbial anabolism, principally the accumulation of microbial metabolic products and necromass, plays a critical role in soil C storage (Schaeffer et al, 2015;Schimel and Schaeffer, 2012). These new findings suggest labile soil C, which is the dominant C and energy source for soil microorganisms (Haider, 1992;Kiem and Kögel-Knabner, 2003), could be an important determinant in the creation of stable SOC.…”