Tree regeneration retards decomposition in a temperate mountain soil after forest gap disturbance Mathias Mayer * , Bradley Matthews, Christoph Rosinger, Hans Sand  en, Douglas L. Godbold, Klaus Katzensteiner Institute of Forest Ecology, Department of Forest and Soil Sciences, University of Natural Resources and Life Sciences Vienna (BOKU), Peter-Jordan Straße 82, 1190 Wien, Austria article info Article history: Received 23 March 2017 Received in revised form 26 July 2017 Accepted 10 September 2017 Keywords: Forest disturbance Heterotrophic soil respiration Soil carbon loss Soil CO 2 efflux Soil enzymes Soil organic matter abstract Disturbances significantly affect the carbon (C) cycle of forest ecosystems. Surviving trees from an un- derstory layer are recognized to play an important role in post-disturbance C dynamics. However, their influence on decomposition of soil organic matter (SOM), an important ecosystem C pool, has yet to be rigorously investigated. Over four consecutive years, we investigated the effects of advance spruce regeneration on decomposition processes in a mountain soil after forest gap disturbance. Gap distur- bance was accomplished by clear cut harvest and decomposition was assessed by combining measure- ments of soil CO 2 efflux, heterotrophic respiration, soil enzymes activity, and mass loss from standardized litter bags. Soil CO 2 efflux showed no response to gap formation, independent of whether regeneration was present or absent, indicating that reduced autotrophic respiration was offset by accelerated heterotrophic respiration from decomposing microbes. Incubation studies revealed no effects of gap disturbance on heterotrophic respiration and its temperature sensitivity under controlled lab conditions. Since potential enzyme activities, at a given temperature, did not respond to gap formation either, it appears that neither C nor other nutrient limitations of the decomposing microbes changed in this SOM-rich mountain forest soil after disturbance. Mass loss of standardized litter was ~5% higher after gap formation in plots without regeneration, while under the regeneration it remained at control stand levels. Our findings indicate that canopy removal by gap disturbance lead to an increase in decompo- sition, primarily due to warmer soil conditions. However, an established regeneration retards decom- position due to its modulating effect on soil temperature. Our study therefore shows that facilitating regeneration pre-disturbance can reduce post-disturbance soil C losses from decomposition. © 2017 Published by Elsevier Ltd. 1. Introduction Forests of the northern hemisphere store large amounts of carbon (C) and thus represent a globally important C sink (Goodale et al., 2002; Luyssaert et al., 2010). Natural and man-made distur- bances can, however, significantly alter the C balance and the C sequestration potential of these ecosystems (Kowalski et al., 2004; Kurz et al., 2008; Amiro et al., 2010; Paul-Limoges et al., 2015). Beside the changes in ecosystem CO 2 fluxes associated with photosynthesis and autotrophic respiration, disturbances can impact heavily on the CO 2 efflux coupled to the decomposition of soil organic matter (SOM). Although disturbance type (e.g. clear cut harvest, bark beetle attack, fire), severity (e.g. single tree mortality, stand replacing mortality), and time since disturbance (e.g. weeks, decades) might influence decomposition differently, recent meta- analyses have shown that disturbances generally cause long- lasting C losses from forest soils (Nave et al., 2010; Thom and Seidl, 2015; Zhang et al., 2015). If, and to what extent a decrease in soil C stocks is actually related to altered decomposition rates and/or changes in C input to soil is, however, still uncertain (Yanai et al., 2003; Spielvogel et al., 2006; Christophel et al., 2015). Furthermore, how the post-disturbance soil C pool is influenced by surviving sub-canopy trees has yet to be addressed in detail. Live tree legacies are of considerable importance for ecosystem resilience (Seidl et al., 2014). Surviving trees from a sub-canopy layer (e.g. seedlings and saplings from regeneration, sub- * Corresponding author. Tel.: 0043 1 47654 91243. E-mail addresses: mathias_mayer@gmx.at (M. Mayer), bradley.matthews@boku. ac.at (B. Matthews), christoph.rosinger@boku.ac.at (C. Rosinger), hans.sanden@ boku.ac.at (H. Sand en), douglas.godbold@boku.ac.at (D.L. Godbold), klaus. katzensteiner@boku.ac.at (K. Katzensteiner). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio https://doi.org/10.1016/j.soilbio.2017.09.010 0038-0717/© 2017 Published by Elsevier Ltd. Soil Biology & Biochemistry 115 (2017) 490e498