Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Food for microorganisms: Position-specific 13 C labeling and 13 C-PLFA analysis reveals preferences for sorbed or necromass C Carolin Apostel a,b,⁎,1 , Jennifer Herschbach c,1 , Ezekiel K. Bore b , Sandra Spielvogel b,c , Yakov Kuzyakov a,b,d , Michaela A. Dippold a a Department of Soil Science of Temperate Ecosystems, University of Goettingen, Goettingen, Germany b Department of Agricultural Soil Science, University of Goettingen, Goettingen, Germany c Institute of Geography, Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland d Institute of Environmental Sciences, Kazan Federal University, 420049 Kazan, Russia ARTICLE INFO Handling Editor: Bernd Marschner 1. Introduction Sorption of low molecular weight organic substances (LMWOS) to soil particles is a key mechanism of soil organic matter (SOM) stabili- zation (Guggenberger and Kaiser, 2003): 70–100% of SOM in mineral horizons is stabilized within mineral-organic associations (Christensen, 2001; Schmidt et al., 2011; Sollins et al., 2007, 1996; von Lützow et al., 2006) and turnover of mineral-associated C is approximately four times slower than that of dissolved organic C (DOC) (Baisden et al., 2002; Kögel-Knabner et al., 2008). This is often attributed to a decrease of SOM accessibility for enzymes and microorganisms (Vieublé Gonod et al., 2006), which are the most important drivers of C dynamics in soil (Kögel-Knabner, 2002). Indeed, sorption strengths of the three amino acids lysine, glycine and glutamate were negatively correlated with their microbial utilization (Jones and Hodge, 1999). In this study, each of the amino acids were added as solutions to the soil and the remaining amounts in the solutions (lysine < glycine < glutamate) were inverse to the pattern observed in CO 2 (glutamate > glycine > lysine). Some microbial groups colonize mineral surfaces, providing them with better access to sorbed SOM (Frey et al., 1999). These microbial groups use a higher proportion of absorbed C for anabolic activities, due to their need to produce larger amounts of polysaccharides and glycoproteins for biofilms and/or hyphae compared to free-living mi- croorganisms (Joergensen and Wichern, 2008; Six et al., 2006). On the other hand, due to the proximity to mineral surfaces, the C contained in these biofilms and the microorganisms themselves can in turn be sta- bilized by sorption to the mineral surfaces after cell death. However, mechanisms underlying the stabilization by sorption and the sub- sequent substance reutilization are difficult to study (Barré et al., 2014) and are therefore not yet fully understood (Kleber et al., 2015). The majority of experiments studying the impact of sorption on SOM sta- bilization were simplified either by being carried out a) in suspension (Dashman and Stotzky, 1982), b) with pure minerals (Mikutta et al., 2007) and/or c) with individual bacterial strains (Cai et al., 2011). Even within these simplified experiments, results varied from no effect on mineralization to a complete cessation of biodegradation (Barré et al., 2014). Therefore, it is necessary to measure not only the net effect of sorption on SOM mineralization, but to also uncover the metabolic mechanisms that drive SOM protection by sorption. Labeling with position-specific isotopic tracers permits not only the quantification of tracer uptake/mineralization, but also enables iden- tification of the underlying metabolic pathways by comparing the possible metabolic transformations for each applied tracer (Apostel et al., 2015; Dijkstra et al., 2015, 2011a, 2011c; Dippold and Kuzyakov, 2016). For example, alanine position C-1 will be oxidized first after uptake in glycolysis, while positions C-2 and C-3 will be oxidized either in the citric acid cycle or partially used for biomass production (Caspi et al., 2014). By identifying a decrease in C-2 and C-3 mineralization, position-specific labeling revealed that sorption not only reduces mi- crobial uptake from soil, but also is facilitated by more efficient mi- crobial metabolism (Dippold et al., 2014; Apostel et al., 2017). How- ever, it is as yet unknown whether a) this metabolic shift affects the whole microbial community, b) sorbed substrates are predominantly taken up by specific microbial groups with more efficient metabolisms or c) specific microbial groups, which predominantly take up sorbed substances, shift their metabolism when using sorbed substrates. To analyze whether the metabolic shift induced by sorption is driven by the whole community or by microbial specialists, it is http://dx.doi.org/10.1016/j.geoderma.2017.09.042 Received 28 February 2017; Received in revised form 22 September 2017; Accepted 29 September 2017 ⁎ Corresponding author at: Department of Soil Science of Temperate Ecosystems, University of Goettingen, Goettingen, Germany. 1 Equal contributions. E-mail address: carolinapostel@yahoo.com (C. Apostel). Geoderma 312 (2018) 86–94 0016-7061/ © 2017 Elsevier B.V. All rights reserved. MARK