Biotic and Abiotic Controls Over Ecosystem Cycling of Stable Natural Nitrogen, Carbon and Sulphur Isotopes

Abstract

In forest ecosystems, physical, chemical and biological processes which regulate the uptake and flow of matter cause differences in the isotopic signatures of N, C and S compounds due to fractionation. Thus, tracing processes of isotopic fractionations in forest ecosystems can help identify transfer pathways and capacities. Atmospheric nitrogen pollution has been recognised as the cause for formerly N-limited forests to approach N saturation (Aber et al. 1989, 1998). The increase in N availability in forest ecosystems may have consequences for net ecosystem productivity on a local scale, but also for the carbon budget of terrestrial ecosystems on a global scale (Schulze 1994; Schimel 1995). Clearly, knowledge about the amount of N being deposited and cycling through forest ecosystems is of paramount importance for the carbon assimilation of the terrestrial biosphere (Lloyd and Farquhar 1996). In a similar manner, information on stable sulphur isotopes is of importance regarding pollution inputs and their effect on ecosystem health (Krouse 1989; Gebauer et al. 1994).