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

Abstract

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 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 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.

Carbon and Nitrogen Pools and Fluxes in Adjacent Mature Norway Spruce and European Beech Forests

“…Contrary to previous paired studies (e.g., [13]), we did find a higher C stock in the forest floor of the beech stand compared to the spruce stand. In an earlier study, we documented partial loss of forest floor C in the spruce stand as a result of recovery from acidification [54]. In the early 1990s, forest floor C mass was 58 t·ha −1 , comparable to the current beech stand, but it has since declined by 47%.…”

“…This assumption might be violated, given the significant decline of forest floor C in the spruce stand noted above [54]. It is difficult to predict whether the spruce soil is currently in a steady state; thus, further calculation might be biased by the annual net loss of C from soil.…”

Nitrogen deposition contributes to soil acidification in tropical ecosystems

“…Although SO 2 emissions have declined across Europe and eastern North America since the 1990s (Oulehle et al, 2011) and in China since 2005 (Fang et al, 2013), the contribution…”

Evidence that acidification‐induced declines in plant diversity and productivity are mediated by changes in below‐ground communities and soil properties in a semi‐arid steppe

“…It has been suggested that soil acidification is a major problem in Chinese grassland and agricultural systems (Zhao et al 2009;Yang et al 2012), although the rate of acid deposition has reduced across Europe and North America since the 1990s (Oulehle et al 2011). For instance, anthropogenic acid deposition decreased soil pH by 0.63 units, on average, in the surface soil layer across northern China's grasslands during the last two decades (Yang et al 2012).…”