Major changes in forest carbon and nitrogen cycling caused by declining sulphur deposition

Sulphur ( S ) and nitrogen ( N ) deposition are important drivers of the terrestrial carbon ( C ) and N cycling. We analyzed changes in C and N pools in soil and tree biomass at a highly acidified spruce site in the C zech R epublic during a 15 year period. Total S deposition decreased from 5 to 1.1 g m −2  yr −1 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 −2  yr −1 , 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 −2  yr −1 at 90 cm) in the 1990s to almost no leaching (<0.02 g  N  m −2  yr −1 ) 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 −1 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 compared 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.