Please cite this article in press as: Casals, P., et al., Root decomposition in grazed and abandoned dry Mediterranean dehesa and mesic mountain grasslands estimated by standard labelled roots. Agric. Ecosyst. Environ. (2010), doi:10.1016/j.agee.2010.10.013 ARTICLE IN PRESS G Model AGEE-3735; No. of Pages 7 Agriculture, Ecosystems and Environment xxx (2010) xxx–xxx Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee Short communication Root decomposition in grazed and abandoned dry Mediterranean dehesa and mesic mountain grasslands estimated by standard labelled roots Pere Casals a,b,∗ , Jordi Garcia-Pausas a,b , Francesc Montané a , Joan Romanyà c , Pere Rovira a a Centre Tecnològic Forestal de Catalunya (CTFC), Ctra de St. Llorenc ¸ de Morunys, km 2, 25280 Solsona, Spain b Fundación Centro de Estudios Ambientales del Mediterráneo (CEAM), Charles Darwin 14, Parc Tecnològic, 46980 Paterna, Spain c Dept. de Productes Naturals, Biologia Vegetal i Edafologia, Universitat de Barcelona, Av. Joan XXIII s/n, 08028 Barcelona, Spain article info Article history: Received 2 August 2010 Received in revised form 19 October 2010 Accepted 20 October 2010 Available online xxx Keywords: 13 C 15 N Grazing exclusion Subalpine grasslands Shrub encroachment Tracer techniques abstract Because root turnover represents the major source for building up soil organic matter in ecosystems with high belowground allocation, like grasslands, sensitive analyses on root decomposition rates may contribute to point out the effect of grazing abandonment on soil C and N dynamics. The objective was to detect changes in root C and N mineralization due to pasture abandonment in Mediterranean dehesa mountain grasslands. Root decomposition was estimated by field incubation of 13 C- and 15 N-labelled wheat roots mixed with unlabelled soil over one year at 5 cm depth in grazed and short-term excluded grasslands in three contrasting situations: (i) a Mediterranean dehesa, (ii) altimontane and (iii) sub- alpine sites. In addition, the long-term effect of grazing abandonment was estimated in a subalpine shrub encroached site. Overall, root decomposition rates decrease from Mediterranean to mountain sites. Moreover, on mountain sites, either grazing exclusion or shrub encroachment reduced 13 C losses from root–soil bags in about four to seven percentage units; in contrast, such an effect was not detected in the Mediterranean grassland. The dynamics of 15 N derived from root–soil bags was site-dependent without a clear pattern related to site climate or grazing abandonment. In general, the fate of mineralized root-N, leached or immobilized in the surrounding soil, seems to be related to soil variables such as the C:N ratio. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Abandonment of extensive managed agricultural and pasture lands is widespread across Europe (European Environment Agency, 1999) and is expected to continue in the years to come (Rounsevell et al., 2006). Hence, understanding how abandonment influences C and N dynamics in grasslands in different climates is essential for clarifying the contribution of these ecosystems to the global C budget and the N cycle. Short-term changes in soil C as a conse- quence of grazing abandonment are difficult to detect because of the large size of the soil organic matter pool as compared to the small changes and high spatial variability (Conant and Paustian, 2002; Smith, 2004). In the long term, grazing abandonment usu- ally involves a change in the dominant functional groups and often leads to shrub encroachment (Archer, 1995). Changes in soil C because of shrub encroachment are easier to detect but not exempt from controversy (Jackson et al., 2002; Montané et al., 2007). In ecosystems with high belowground allocation, such as grasslands, ∗ Corresponding author at: Centre Tecnològic Forestal de Catalunya (CTFC), Ctra de St. Llorenc ¸ de Morunys, km 2, 25280 Solsona, Spain. Tel.: +34 973 48 17 52; fax: +34 973 48 04 31. E-mail address: pere.casals@ctfc.cat (P. Casals). roots may represent the main source for building up soil organic matter (Balesdent and Balabane, 1996; Pi ˜ neiro et al., 2006; Rasse et al., 2005). Therefore, as the decomposition drives C cycling and N dynamics, a more thorough understanding of the factors regulat- ing root decomposition will improve our ability to model global C dynamics, and predict the effects of future climate and other global changes on biogeochemical cycles (Silver and Miya, 2001). By itself, root quality is a good predictor of root decomposition (Aerts, 1997; Silver and Miya, 2001), but this prediction capacity improves if root quality is combined with environmental vari- ables (Silver and Miya, 2001). At local level, fine root decay may be sensitive to small changes in microclimate or in soil microbial communities caused by disturbances or land use changes. Higher standing necromass and litter after grazing abandonment reduces soil temperature and may increase soil water content (Naeth et al., 1991; Rosset et al., 2001). Although (Giese et al., 2009) did not find any effect related to grazing abandonment, soil water con- tent was the best parameter to explain root decomposition in semi-arid mongolian grasslands. Thus, we hypothesized that, in water-limited Mediterranean grasslands, root decomposition rates would increase after grazing abandonment because of the higher soil water availability. In contrast, in mesic mountain sites with cold climates, a decrease in root decomposition rates is expected after abandonment because of soil temperature reductions (Rosset 0167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2010.10.013