Technogenic parent materials drive Technosols humus system formation on an industrial brownfield

•Three different humus systems were found on a former coking plant area.•Technogenic parent material was the main factor driving humus system formation.•The presence of zoOH horizon was not influenced by the litter type.•Organic horizon chemical properties depended on both litter and soil substrate.•zoOH horizon improved Technosol humipedon’s chemical quality. The study of humus systems provides a lot of information on soil ecological functioning, such as fauna functional community, soil nutrient status, organic matter decomposition, and microbial food web. Technosols have highly heterogeneous physicochemical properties and so far, their humus systems are poorly described and the mechanisms behind their formation are still largely to be studied. In order to fill this knowledge gap, this study was conducted to get better understanding of the factors involved in the development of different Technosols humus system after brownfield rehabilitation. Our sampling strategy followed a pollution diagnostic report and allowed to cope with the spatial heterogeneity of the studied area. We studied 24 soil profiles and identified 3 different humus systems on two different rehabilitation substrates: 7 Mull, 3 Amphi and 14 Techno-moder. Our results showed that the development of zoOH horizon resulted from the nature of underlying horizons but not from the main litter type. On Techno-moder, the accumulation of organic matter in the form of fecal pellets as a zoOH horizon resulted in an increase of 130% of nitrogen on soil surface. Contrary to what was expected, we did not find a threshold of heavy metals leading to the formation of zoOH horizon, with highest lead concentration found in the A horizon of Mull systems. Transmission electron microscopy (TEM) analysis showed that neoformed organic matter is biochemically stabilized as condensed polyphenols and melanized hyphae. EDX analysis revealed that such ultrastructures are associated with Ca and Zn that may explain their stability on the Technosols surface. Development of zoOH horizon on Technosols built from coking plant by-products allowed to improve soil surface chemical and biological properties. The evolution of the humus systems as a result of vegetation succession and microbial community composition should be further studied to specify the mechanisms involved in Technosols humus system dynamics.