Biogeochemistry The Jena Perspective

In the late 20th century, biogeochemistry emerged as a new discipline in which the biological, physical, and human sciences collaborate (CGCR, 1999; Schlesinger, 1997). Biological, because the chemical cycles of the planet are mediated by life (Table 1). Physical, because of the strong coupling between climate and atmospheric composition so evident in the glacial-interglacial record of the ice cores (Fig. 1). And, human, because of the massive human disruption of the planet's carbon and nitrogen cycles by fossil fuel burning (which produces CO, and a range of volatile nitrogen compounds) (Fig. 2).

From the three figures, one gets an overview of the way in which the field of biogeochemistry has emerged. The evidence for the importance of biology in the composition of the atmosphere (Fig. 2) was deduced from geochemical measurements of air enabled by advances in analytical technology. The chemistry of the atmosphere and the discipline of atmospheric chemistry provided a view of the biosphere not accessible from "within" the discipline. The atmosphere reflects biotic processes operating over "deep" time as well as processes operating on rapid time scales (especially with respect to the oxidized N species). Some compounds, especially the hydrocarbons, may reflect plant-insect coevolution, and so to understand the atmosphere requires a deep understanding of biology. When insights into atmospheric chemistry were combined with emerging ecosystem studies of nitrogen and other elements (e.g., Vitousek and Reiners, 1977), a paradigm emerged that enriched both ecology and geophysics (Andreae and Schimel, 1989).

The realization that ecosystem biogeochemistry and climate were dynamically coupled was nascent for most of the 20th century. The ice-core records showing the coordinated rhythm of temperature, C02, and methane provided conclusive evidence of interactions (Fig. 2). The ice cores show coupled changes in trace gases and climate. They preserve a tantalizing body of information about leads, lags, and amplification that is not yet fully unravelled. While variations in C02 are strongly governed by changes in ocean circulation, mass balance considerations and isotopes suggest land-ecosystem changes as well (Indermuhle et ai, 1999). Climate effects on terrestrial biogeochemistry are demonstrated by the patterns in methane (produced in terrestrial wetlands and ungulate mammals) and nitrous oxide. High-resolution records showing high-frequency changes in ice cores, and detailed records of the Holocene provide information on timescales tractable, or nearly so, in analysis using today's biogeochemical models. Again, the perspective from geophysical records provides a view of ecosystem processes different from, and most strongly complementary to, the paradigms emerging from within the discipline.

The scientific community was galvanized by the Mauna Loa curve of increasing carbon dioxide and the political ramifications of this scientific result will echo for the foreseeable future (Benedick, Chapter 26 of this volume). Geophysical measurements provide a trans-disciplinary view of human processes. Since biogeochemistry has a "basic science" character and remains concentrated in academia, the carbon and nitrogen cycles would be of far less interest without the challenges of carbon and climate change, acid rain, and tropospheric ozone increase. The Mauna Loa curve challenges both the policy-relevant and intellectual sides of biogeochemistry. The policy side is obvious—the rate of increase in atmospheric C02 is the index of humanity's export of carbon to the atmosphere.

Scientifically, the fraction of C02 released to the atmosphere that remains as C02 in the air (about half) is not yet explained on the basis of incontrovertible measurements. While the holy grail of explaining the "missing sink" grows asymptotically closer, the political stakes and hence the standard of proof required are growing. The interan-nual variability of the growth rate of C02 gives evidence of climate-carbon interactions. Subtle year-to-year variations in the increase in C02 reflect changes in land and ocean uptake. The measurement and modeling tools to understand these changes are emerging and provide a direct means of understanding how climate affects the carbon system at large scales. Changes in the carbon system are reflected also in changes in the pole-to-pole gradient of C02. That gradient reflects the balance of sources and sinks on large scales. Because the equilibration time (the interhemispheric transport time) is about a year, changes in the gradient are another source of information about interannual variability. The seasonal cycle of C02 provides information about the seasonal activity of the biosphere. Because the phase and amplitude of the seasonal cycle vary spatially (Fig. 3), they provide rich information about land ecosystems. To date, we cannot fully separate changes in carbon uptake (photosynthesis) and release (respiration) to provide unique explanations for the seasonal cycle and its variation. This remains a research challenge.

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