Soil Biology and Biochemistry 151 (2020) 108036 Available online 8 October 2020 0038-0717/© 2020 Elsevier Ltd. All rights reserved. Mineralization of organic carbon and formation of microbial biomass in soil: Effects of clay content and composition and the mechanisms involved Fatemeh Rakhsh a, * , Ahmad Golchin a , Ali Beheshti Al Agha b , Paul N. Nelson c a Soil Science Department, Faculty of Agriculture, University of Zanjan, Zanjan, Iran b Soil Science Department, Faculty of Agriculture, Razi University, Kermanshah, Iran c College of Science and Engineering, James Cook University, Cairns, Australia A R T I C L E INFO Keywords: Biomass carbon Carbohydrate Cation exchange capacity Oxyhydroxides Specifc surface area Water holding capacity ABSTRACT Mineralization of soil organic carbon and CO 2 emission from the soil is slowed by interactions between organic matter and minerals. The main minerals involved are clay minerals and oxides but there is limited understanding of their effects when combined, as occurs in soil. We aimed to determine the effects of clay content and composition on organic carbon stabilization in soil, and the mechanisms involved. This was achieved by studying the decomposition of alfalfa residues in artifcial soils made from quartz sand and kaolinite with and without additions of the non-layered colloids (NLCs) goethite, manganese oxide or imogolite. The artifcial soils were inoculated with microbes from natural soil and incubated at 23 ◦ C in the dark at 60% of water holding capacity for 180 days. With increasing contents of clay and NLCs, organic carbon mineralization decreased, whereas carbohydrate and microbial biomass carbon contents increased. Of the NLCs, goethite had the least effect and imogolite the greatest effect on carbohydrate content. The effects of the treatments on mineralization and car- bohydrate content were explained mostly by specifc surface area (> 83% of variation), presumably due to the effects on sorption. The effects of the treatments on microbial biomass were related to the volume of habitat (water-flled pore space) and availability of substrate (infuenced by sorption). These results showed that clay content and composition infuenced the stabilization of soil organic carbon mostly through the supply of surfaces for sorption reactions rather than via interactions unique to particular colloids. 1. Introduction Decomposition of organic matter in terrestrial ecosystems is a major source of atmospheric CO 2 (Heimann and Reichstein, 2008; Schmidt et al., 2011). CO 2 accounts for 60% of the greenhouse effect and con- cerns about global climate change have increased interest in clarifying our understanding of organic matter decomposition and retention in soils (Kleber et al., 2011; Cotrufo et al., 2015). Decomposition is slowed through the interaction of organic matter with soil mineral particles (Baldock and Skjemstad, 2000a; Kalbitz et al., 2003; K¨ ogel-Knabner et al., 2008). Stabilization of organic matter in this way also supports plant growth and other soil functions, due to the positive impact of organic matter on the physical, chemical, and biological properties of soil. One mechanism of interaction is the sorption of organic matter, which has been shown to decrease its availability for microbial decomposition and signifcantly reduce carbon mineralization rates (Sollins et al., 1996; Guggenberger and Kaiser, 2003). Phyllosilicate clays and metal oxides (including oxyhydroxides, hy- droxides and hydrous oxides) are important minerals for stabilizing organic matter in soils and for retarding the mineralization of organic carbon (Mikutta et al., 2007; Saidy et al., 2012). It is typically diffcult to remove organic matter absorbed to mineral surfaces, indicating strong organic matter-mineral bonds (Butman et al., 2007; Kaiser and Gug- genberger, 2007). Therefore, organic matter absorbed to clay minerals and metal oxides decomposes more gradually and to a lesser extent than free organic matter (Mikutta et al., 2007; Schneider et al., 2010). Hy- drous Fe and Al oxides are particularly effective in the absorption and stabilization of organic matter in soils (Kaiser and Guggenberger, 2003; Kaiser et al., 2007; Goodrick and Nelson, 2018). For example, complexes of organic matter-ferrihydrite have been observed to play a key role in immobilizing water soluble organic matter and protecting it against microbial decomposition (Eusterhues et al., 2011; Henneberry et al., 2012). Metal oxides have a major impact on soil properties and are ubiquitous in soils. They can have high surface area and tend to be most * Corresponding author. E-mail address: rakhsh.fatemeh@znu.ac.ir (F. Rakhsh). Contents lists available at ScienceDirect Soil Biology and Biochemistry journal homepage: http://www.elsevier.com/locate/soilbio https://doi.org/10.1016/j.soilbio.2020.108036 Received 14 May 2020; Received in revised form 28 September 2020; Accepted 2 October 2020