The Carbon and Nitrogen Cycle of Forest Ecosystems

Abstract

Our understanding of the biology of major biogeochemical cycles came initially from, and is still based upon, field observations (Bolin et al. 1979; Clark and Rosswall 1981; Apps and Price 1996). This is in contrast to very advanced models, which explore the physics of the climate system and are based on laws of physics or chemistry with a mechanistic understanding of the underlying processes (Houghton et al. 1996; Bengtsson 1999). For the biologist, the responses of organisms reach far beyond physicochemical reactions, and they include genetically regulated changes in physiological pathways or activation of enzyme systems as part of acclimations and adaptations that are coupled with climate and species composition changes. Generic predictions thus remain elusive because there are too many species and pathways. Although climate greatly influences the biogeochemical cycles, models that include biology thus remain at a correlative level. Moreover, the cycling of elements like carbon (C) cannot readily be separated from the abundance, state and cycles of other elements, especially nitrogen (N) (Schulze et al. 1994) which, in turn, is tied to the cycling of other elements (Ulrich 1987). Nevertheless, detailed knowledge of the biology of C cycling and that of other major and minor elements is urgently needed because the Kyoto Protocol demands strategies to balance industrial emissions by biological C fixation (WBGU 1998; IGBP 1998). By this protocol, mankind is taking a first step to deliberately engineer the biology of the global C cycle; but without full understanding of the underlying processes, there is a risk of serious deleterious side effects (Schellnhuber and Wenzel 1998; Schellnhuber 1999).