Estimating the critical N:C from litter decomposition data and its relation to soil organic matter stoichiometry G.I. Ågren a, * , R. Hyvönen a , S.L. Berglund a , S.E. Hobbie b a Department of Ecology, Swedish University of Agricultural Sciences, P.O. Box 7044, SE-750 07 Uppsala, Sweden b Department of Ecology, Evolution and Behavior, University of Minnesota, Saint Paul, MN 55108, USA article info Article history: Received 21 February 2013 Received in revised form 27 June 2013 Accepted 6 September 2013 Available online 20 September 2013 Keywords: Carbon Decomposition Litter Nitrogen Soil organic matter Stoichiometry abstract Carbon (C) and nitrogen (N) interact strongly during decomposition of litters with repercussions for stoichiometry of soil organic matter (SOM). A characteristic parameter describing this interaction is the critical N:C ratio, r c , i.e. the litter N:C ratio at which net N mineralisation commences. We show here how r c can be estimated from time series of decomposition and use four large data bases to investigate how environment and litter properties determine r c . We find that r c increases with increasing initial litter N:C and N fertilisation. The critical N:C responds also to large scale variations in environment but it is not clear which environmental variable drives this variability. With increasing r c we find an increase in SOM N:C. SOM N:C increases more rapidly than r c , which can be explained if the decrease in substrate quality during decomposition is taken into account. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbon (C) and nitrogen (N) interact strongly during decom- position of litters with repercussions for the mass and stoichiom- etry of soil organic matter (SOM). Much attention has, therefore, been devoted to understand and predict the consequences of this interaction. The question in most studies has been how the CeN- interaction regulates the rate of release of C and N from litters and SOM, and it has been extensively shown that there is a strong positive correlation between N:C in litters and the rate of litter mass loss (e.g. Melillo et al., 1982; Hobbie, 2005; Cornwell et al., 2008). As a consequence, N:C or N:lignin is often used as an in- dex of quality or decomposability of litter (e.g. Aerts, 1997). On top of litter quality, climate plays a major role for decomposition rates (e.g. Aerts, 1997). Other environmental factors such as a possible interaction between physical and chemical litter properties and the decomposer community also contribute to modifying rates. However, C and N differ in one important respect in that the mass of C is constantly decreasing while in initially N-poor litters the mass of N will increase up to some critical N:C ratio (r c ) and thereafter decrease; in sufficiently N-rich litter this decrease will commence immediately. This relation between C and N is well established both empirically (e.g. Aber and Melillo, 1980; Moore et al., 2006, 2011; Parton et al., 2007; Manzoni et al., 2008) and theoretically (Ågren and Bosatta, 1998). The relation is simple and depends on only three system properties: (i) initial N:C ratio of the litter (r 0 ), (ii) N:C ratio of the decomposer community (r d ), and (iii) carbon-use efficiency or production-to-assimilation ratio of the decomposer community (e 0 ); the critical N:C ratio can be shown to be the product of e 0 and r d (r c ¼ e 0 r d )(Ågren and Bosatta, 1998). An important aspect of this relation is that it is independent of the rate of decomposition; i.e. climate plays no role to the extent that it does not affect r d or e 0 (see Discussion). Although the masses of C and N in the decomposer biomass are small compared with SOM C and N (3 and 5%, respectively, in the data from Cleveland and Liptzin, 2007), the decomposer stoichiometry is important because de- composers are the gateway through which SOM is processed and therefore leave a strong imprint on SOM stoichiometry. The overall SOM stoichiometry will then be determined by a balance between plant material that has still escaped decomposers and material that has been processed to different degrees. On top of that, there can also be purely chemical processes that attach or detach N from C skeletons. A problem with the parameters coupling C and N is the strong negative correlation between r d and e 0 observed in estimates and predicted by theory (Manzoni et al., 2008, see also below), making * Corresponding author. Tel.: þ46 18 672449. E-mail address: goran.agren@slu.se (G.I. Ågren). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2013.09.010 Soil Biology & Biochemistry 67 (2013) 312e318