Contents lists available at ScienceDirect Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil Foliage C:N ratio, stage of organic matter decomposition and interaction with soil affect microbial respiration and its response to C and N addition more than C:N changes during decomposition Veronika Jílková a , Petra Straková b,c , Jan Frouz a,d, ⁎ a Biology Centre, Czech Academy of Sciences, Institute of Soil Biology and SoWa RI, Na Sádkách 7, České Budějovice CZ-37005, Czech Republic b Department of Forest Sciences, Peatland Ecology Group, University of Helsinki, Latokartanonkaari 7, Helsinki FI-00014, Finland c Natural Resources Institute Finland (Luke), Latokartanonkaari 9, Helsinki FI-00790, Finland d Charles University in Prague, Institute for Environmental Studies, Benátská 2, Prague CZ-12801, Czech Republic ARTICLE INFO Keywords: Light fraction Heavy fraction Litter Nutrients Carbon dioxide Spoil heaps ABSTRACT How litter at various stages of decomposition reacts to C and N additions is unclear. Here we used five substrates (litter, fermentation [Oe] layer, bulk soil, and the light fraction [LF] and heavy fraction [HF] of SOM) obtained from sites supporting five plant monocultures (Alnus glutinosa, Quercus robur, Salix caprea, Calamagrostis epigejos, or Picea omorica) with foliage C:N ratios ranging from 17 to 48. These plant-specific communities were ex- perimentally planted on a post-mining heap and had affected the substrates used in this study for 40 years. Soils and other environmental factors were similar among the sites. Substrates were incubated for 3 weeks without nutrient addition or with C (glucose) or N (ammonium nitrate) addition, and microbial respiration was de- termined weekly. Substrate C:N ratios were determined at the start of the incubation and were highest for litter followed by Oe layer > LF > bulk soil and HF. Foliage C:N ratio was a better indicator of microbial respiration than the substrate C:N ratio, suggesting that the foliage C:N ratio reflected unmeasured leaf properties that determined microbial respiration. Respiration was highest in the litter followed by Oe layer > bulk soil > LF > HF. C addition increased respiration of the bulk soil (+39%), LF (+48%), and HF (+72%). Priming of SOM respiration was therefore higher in substrates with less available C. N significantly increased respiration of litter (+19%) but decreased respiration of bulk soil (-18%). The difference in respiration of HF vs. bulk soil following N addition suggested that, in addition to the stage of decomposition, environmental properties present in bulk soil but absent in HF may cause the reduction in respiration after N addition to bulk soil. Overall, the results indicate that differences in the contents of SOM fractions among soils will affect the responses of those soils to C and N additions. 1. Introduction The carbon dioxide (CO 2 ) concentration in the atmosphere has in- creased to almost 400 ppm (IPCC, 2014), and higher carbon (C) inputs to soils are thus expected due to increased litter production and rhi- zodeposition (Cotrufo et al., 1998a, 1998b; Coûteaux et al., 1999; Norby et al., 2005; Phillips et al., 2006). Free-air CO 2 enrichment stu- dies corroborate these expectations in that elevated CO 2 (ambient +200 ppm) causes significant increases in litterfall mass and/or root increment (e.g., Allen et al., 2000; Hoosbeek and Scarascia-Mugnozza, 2009; Lukac et al., 2009). Moreover, most ecosystems are experiencing increased inputs of anthropogenically derived nitrogen (N) (Galloway et al., 2008; Schlesinger, 2009; Vitousek et al., 1997). Several studies show that a large-scale N input to soil by enhanced fertilization and atmospheric deposition is likely to reduce soil respiration (Janssens et al., 2010; Schlesinger and Andrews, 2000). When sites are not N- limited, a long-term N addition also decreases soil respiration (Guo et al., 2017), but here mechanisms are likely to be more complex and include soil acidification and a shift in microbial community composi- tion. Because C storage or content is nearly three times greater in soil than in aboveground biomass and is approximately two times greater than in the atmosphere (Eswaran et al., 1993; Schlesinger, 1977), an important question is “How will soils react to such high C and N in- puts?” An understanding of the response of soil organic C (SOC) to https://doi.org/10.1016/j.apsoil.2020.103568 Received 23 October 2019; Received in revised form 18 February 2020; Accepted 20 February 2020 ⁎ Corresponding author at: Biology Centre, Czech Academy of Sciences, Institute of Soil Biology and SoWa RI, Na Sádkách 7, České Budějovice CZ-37005, Czech Republic. E-mail address: frouz@natur.cuni.cz (J. Frouz). Applied Soil Ecology 152 (2020) 103568 0929-1393/ © 2020 Elsevier B.V. All rights reserved. T