Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material BJO ¨ RN ERHAGEN*, MATS Ö QUIST*, TOBIAS SPARRMAN † , MAHSA HAEI*, ULRIK ILSTEDT*, MATTIAS HEDENSTR € OM † ,J € URGEN SCHLEUCHER ‡ andMATS B. NILSSON* *Department of Forest Ecology & Management, Swedish University of Agricultural Sciences (SLU), Skogsmarksgr€ and, Umea ˚, SE-901 83, Sweden, †Department of Chemistry, Umea ˚ University, Umea ˚ SE-901 87, Sweden, ‡Department of Medical Biochemistry and Biophysics, Umea ˚ University, Umea ˚ SE-901 87, Sweden Abstract The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO 2 concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q 10 in litter and SOM, respectively. The Q 10 of litter decreased with increasing proportions of aromatic and O-aromatic compounds, and increased with increased con- tents of alkyl- and O-alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the 13 C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate. Keywords: CO 2 , CP-MAS NMR, decomposition, forest, litter, organic chemical composition, Q 10 , soil organic matter, tempera- ture sensitivity Received 26 February 2013 and accepted 21 July 2013 Introduction The world’s soils contain around 3000 Gt of carbon (Tarnocai et al., 2009) 40% of which is stored in the northern hemisphere within the Boreal region (IPCC, 2000). Consequently, loss of carbon stored in soil in ter- restrial ecosystems through decomposition may contrib- ute significantly to rising atmospheric carbon dioxide (CO 2 ) levels linked to climate change (Kirschbaum, 2000; Allison & Treseder, 2011). Temperature has a pro- found effect on the microbial decomposition of soil organic matter (SOM), thus more precise knowledge regarding its effects on different carbon compounds is required to predict how global warming will affect soil carbon stocks and associated feedback processes (Frie- dlingstein et al., 2006; Heimann & Reichstein, 2008). General understanding regarding how SOM decom- position is affected by temperature is based on kinetic theory based on Arrhenius-type reactions (Arrhenius, 1889), where decomposition increases with increasing temperature where substrate availability and enzyme activity are not constraints in the reaction rate (Conant et al., 2011). Kinetic theory also suggests that organic compounds with low quality (i.e. a higher activation energy) should experience a higher proportional increase in reaction rate compared with organic com- pounds with high quality (i.e. a low activation energy). In addition, temperature response is not linear with respect to increasing temperature but is expected to be greater at lower temperature ranges as compared to higher temperature ranges (Davidson & Janssens, Correspondence: Bj€ orn Erhagen, tel. +46 090 786 8232, fax +46 0 90 7868163, e-mail: bjorn.erhagen@slu.se © 2013 John Wiley & Sons Ltd 3858 Global Change Biology (2013) 19, 3858–3871, doi: 10.1111/gcb.12342