Linking temperature sensitivity of soil organic matter decomposition to its molecular structure, accessibility, and microbial physiology ROTA WAGAI, AYAKA W. KISHIMOTO-MO, SEIICHIRO YONEMURA, YASUHITO SHIRATO, SYUNTARO HIRADATE andYASUMI YAGASAKI National Institute for Agro-Environmental Sciences (NIAES), Tsukuba, Ibaraki 305-8604, Japan Abstract Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warm- ing. Enzyme-kinetic hypothesis suggests that decomposition of low-quality substrate (recalcitrant molecular struc- ture) requires higher activation energy and thus has greater temperature sensitivity than that of high-quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme-substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low-density fraction (LF) which represents readily accessible, mineral-free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30-days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm À3 ) and bulk soil was measured by solid-state 13 C-NMR. Decomposition Q 10 was significantly correlated with the abundance of aromatic plus alkyl-C relative to O-alkyl-C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respira- tion or biomass. The warming did not significantly change the concentration of biomass C or the three types of solu- ble C despite two- to three-fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C-use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme-kinetic interpretation of widely observed C quality-temperature relationship for short-term decomposition. Factors controlling long-term decomposition Q 10 are more complex due to protective effect of mineral matrix and thus remain as a central question. Keywords: activation energy, carbon molecular structure, carbon use efficiency, density fractionation, dissolved organic matter, enzyme kinetics, soil organic matter, substrate quality, temperature dependency Received 29 August 2012; revised version received 9 November 2012 and accepted 23 November 2012 Introduction Decomposition of soil organic matter (SOM) has signifi- cant impact on land-atmosphere C exchange directly via microbial heterotrophic respiration and indirectly by regulating nutrient availability for primary produc- tion. Global simulation models show dramatically different predictions of the response of soil C pool to future warming (Jones et al., 2005; Friedlingstein et al., 2006) because of our limited understanding on SOM dynamics including its temperature sensitivity (David- son & Janssens, 2006; Kirschbaum, 2006; Von L€ utzow & K€ ogel-Knabner, 2009; Hopkins et al., 2012). The SOM decomposition process is inherently complex due to enormous variations in heterotrophic microorganisms, enzymes they secrete, and their substrates (plant- and microbially synthesized compounds at different decay- ing stages). Moreover, those interactions with soil min- eral particles and aggregates (e.g., sorption on mineral surfaces, and physical protection) create wide arrays of constrains for decomposition (Sollins et al., 1996; Baldock & Skjemstad, 2000; Davidson & Janssens, 2006; Gillabel et al., 2010; Schmidt et al., 2011). As a result, there is still no accepted theory on how temperature controls the overall response of all the substrate- enzyme reactions involved in SOM decomposition (Kirschbaum, 2006; Conant et al., 2011). A leading hypothesis explaining decomposition tem- perature sensitivity is based on thermodynamic princi- ples applied to the enzyme-substrate reactions occurring in soil (Bosatta &  Agren, 1999; Fierer et al., 2005; Davidson & Janssens, 2006; Craine et al., 2010; Conant et al., 2011). The enzyme-kinetic hypothesis Correspondence: Rota Wagai, tel. +81 29 838 8327, fax +81 29 838 8199, e-mail: rota@affrc.go.jp 1114 © 2012 Blackwell Publishing Ltd Global Change Biology (2013) 19, 1114–1125, doi: 10.1111/gcb.12112