Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena Influence of soil forming factors on the molecular structure of soil organic matter and carbon levels Marco A. Jiménez-González a,b, ⁎ , Ana M. Álvarez b , Pilar Carral b , Gonzalo Almendros a a Department of Biogeochemistry and Microbial Ecology, National Museum of Natural Sciences, 28006 Madrid, Spain b Department of Geology and Geochemistry, Autonomous University of Madrid, 28049 Madrid, Spain ARTICLE INFO Keywords: Analytical pyrolysis Carbon sequestration Discriminant analysis Soil organic matter ABSTRACT There is currently an active controversy about the variable influence of the factors involved in the total content and the quality of the soil organic matter (SOM), which translates into its resilience and stability against bio- degradation, and importantly on the rates of release of CO 2 into the atmosphere. The aim of this work is to study the molecular composition of SOM in contrasting environments in order to evaluate the extent to which such conditions may affect SOM characteristics in addition to the levels of soil organic C (SOC). Up to 33 soils from different environmental scenarios of Spain were analysed by pyrolysis combined with gas chromatography mass spectrometry (Py-GC/MS). The 193 major pyrolysis compounds released from the soils were included in a chemostatistical study based in discriminant analysis to assess the impact of classical soil forming factors (i.e., climate, vegetation and geological substrate) in SOM content and composition. Improved van Krevelen diagrams were used to facilitate the recognition of different patterns in SOM composition dependent on soil forming factors. The results showed that the molecular composition of SOM varies systematically according to en- vironmental factors, with a decreasing influence in the order: climate > vegetation > geological substrate. In addition, the total levels of SOM were also different depending on the environmental scenarios on these soils, suggesting both qualitative and quantitative control of soil C sequestration. 1. Introduction The factors involved in the transformation of soil organic matter (SOM) are extremely complex and responsible of its total levels and quality. While some authors highlight the importance of climatic factors in controlling the soil formation processes, others consider the effect of vegetation or geological substrate more important (Duchafour and Jacquin, 1975; Ganuza and Almendros, 2003; Jenny, 1994; Johnson et al., 2011; Towett et al., 2015). In addition, there is still no consensus regarding the quantitative influence of the factors that determine the fact that some soils store more soil organic carbon (SOC) than others, which results in important differences in their potential for sequestra- tion of atmospheric carbon that is currently a subject of great interest (Lal, 2004). At this respect, it is also well known that organo-mineral interactions and microencapsulation of the SOM may play an important role on the physico-chemical protection of the SOM (Simonetti et al., 2017; Song et al., 2012; Spaccini et al., 2002). On the other hand, land use may also affect soil C sequestration processes (Hernández et al., 2019; Pizzeghello et al., 2017; Yazdanshenas et al., 2018). Apart of this and regarding soil C level, it is also relevant to point out that the chemical composition of the SOM may also be an important constraint in its recalcitrancy (Jiménez-González et al., 2017, 2018, 2019). This is traditionally taken into account when referring the melanization of soil horizons, which reflects the fact that, depending on the environmental conditions, the accumulation of humus is variable in quantity and quality (Bockheim and Hartemink, 2017; Di et al., 2019). In this context, the use of analytical degradation techniques for molecular characterization of the SOM, such as Py-GC/MS, may re- present a source of semiquantitative data suitable to explore the mo- lecular composition of the SOM. This technique has a series of ad- vantages such as not requiring chemical pretreatments, or previous SOM isolation with alkaline reagents (Derenne and Quénéa, 2015; Schnitzer and Schulten, 1992). The composition of SOM may represent a reliable repository of environmental information about the influence of soil forming factors, such as climatic conditions, vegetation types and geological composition. Consequently, a major goal of this research is to assess the influence of the above factors on the humification pro- cesses, as reflected by the molecular composition of the SOM, but also to assess the biogeochemical meaning of the variability in the chemical composition of the SOM in terms of the different SOC levels. https://doi.org/10.1016/j.catena.2020.104501 Received 27 June 2019; Received in revised form 23 January 2020; Accepted 28 January 2020 ⁎ Corresponding author. E-mail address: majimenez@mncn.csic.es (M.A. Jiménez-González). Catena 189 (2020) 104501 0341-8162/ © 2020 Elsevier B.V. All rights reserved. T