Major changes in forest carbon and nitrogen cycling caused by declining sulphur deposition FILIP OULEHLE* † , CHRISTOPHER D. EVANS*, JENYK HOFMEISTER ‡ , RADOVAN KREJCI § , KAROLINA TAHOVSKA ¶ , TRYGGVE PERSSON k, PAVEL CUDLIN** andJAKUB HRUSKA † *Centre for Ecology and Hydrology, Bangor, LL57 2UP, UK, †Department of Environmental Geochemistry and Biogeochemistry, Czech Geological Survey, Klarov 3, Prague, 118 21, Czech Republic, ‡Areal COV Horovice, Horovice, 268 01, Czech Republic, §Department of Applied Environmental Science, Stockholm University, Stockholm, 106 91, Sweden, ¶Department of Ecosystem Biology, University of South Bohemia, Branisovska 31, Ceske Budejovice, 370 05, Czech Republic, kDepartment of Ecology, Section of Soil Ecology, Swedish University of Agricultural Sciences, Box 7044, Uppsala, 750 07, Sweden, **Institute of Systems Biology and Ecology, Academy of Sciences of the Czech Republic, Na Sadkach 7, Ceske Budejovice, 370 05, Czech Republic Abstract Sulphur (S) and nitrogen (N) deposition are important drivers of the terrestrial carbon (C) and N cycling. We ana- lyzed changes in C and N pools in soil and tree biomass at a highly acidified spruce site in the Czech Republic during a 15 year period. Total S deposition decreased from 5 to 1.1 g m À2 yr À1 between 1995 and 2009, whereas bulk N deposition did not change. Over the same period, C and N pools in the Oa horizon declined by 116 g C and 4.2 g N m À2 yr À1 , a total decrease of 47% and 42%, respectively. This loss of C and N probably originated from organic matter (OM) that had accumulated during the period of high acid deposition when litter decomposition was suppressed. The loss of OM from the Oa horizon coincided with a substantial leaching (1.3 g N m À2 yr À1 at 90 cm) in the 1990s to almost no leaching (<0.02 g N m À2 yr À1 ) since 2006. Forest floor net N mineralization also decreased. This had consequences for spruce needle N concentration (from 17.1 to 11.4 mg kg À1 in current needles), an increase in litterfall C/N ratio (from 51 to 63), and a significant increase in the Oi + Oe horizon C/N ratio (from 23.4 to 27.3) between 1994 and 2009/2010. Higher forest growth and lower canopy defoliation was observed in the 2000s com- pared to the 1990s. Our results demonstrate that reducing S deposition has had a profound impact on forest organic matter cycling, leading to a reversal of historic ecosystem N enrichment, cessation of nitrate leaching, and a major loss of accumulated organic soil C and N stocks. These results have major implications for our understanding of the controls on both N saturation and C sequestration in forests, and other ecosystems, subjected to current or historic S deposition. Keywords: acidification, carbon, deposition, DOC, forest floor, leaching, nitrogen, nitrogen saturation, soil, sulphur Received 24 January 2011; revised version received 1 April 2011 and accepted 13 May 2011 Introduction Many forest ecosystems in Europe and North America have suffered from soil acidification and ecosystem eutrophication as a result of anthropogenic emissions of SO 2 , NO x and NH 3 . It is now clear that atmospheric deposition pollutant has impacted on the wider biogeo- chemical cycle of forest ecosystems, including the eco- system carbon (C) balance. A majority of temperate ecosystems are nitrogen (N) limited, thus fertilisation by atmospheric N deposition is thought to have increased C storage in biomass and soils of terrestrial ecosystems and thus stimulates C sequestration. Esti- mates of the magnitude of this sink in forests vary con- siderably, from more conservative estimates between 5–75 kg C kg N À1 (Nadelhoffer et al., 1999; Currie et al., 2004; Hyvo ¨nen et al., 2008; De Vries et al., 2009) and up to 500 kg C kg N À1 (Holland et al., 1997; Mag- nani et al., 2007). The increase of N availability may increase net primary productivity and/or decrease decomposition of N-rich litter (Janssens et al., 2010; Liu & Greaver, 2010). While most recent research has focused on the impacts of N deposition on C cycling, there is evidence that acidification [from either sulphur (S) or N deposition] may also impact on a range of key processes, including suppression of litter decomposi- tion. This may occur through reduction in the availabil- ity of C for microbes rather than a direct effect on the microorganisms themselves (Persson et al., 1989; Pennanen et al., 1998). C limitation may be induced by the adverse effect of Al, which can form insoluble Correspondence: Filip Oulehle, tel. + 44 (0) 1248 374539, fax + 44 (0) 1248 362133, e-mails: filip@ceh.ac.uk; filip.oulehle@geology.cz © 2011 Blackwell Publishing Ltd 3115 Global Change Biology (2011) 17, 3115–3129, doi: 10.1111/j.1365-2486.2011.02468.x