Accelerating human impacts on the biosphere increasingly threaten the global life-support systems on which human society depends. This Special Issue of Frontiers – resulting from a series of sessions entitled Coupled Biogeochemical Cycles, held at the 2009 Ecological Society of America (ESA) Annual Meeting in Albuquerque, New Mexico, and sponsored by the National Science Foundation – offers a revised framework for understanding why human activities have had such profound ecological and societal consequences across local to global scales. Bringing together ecologists, biogeochemists, atmospheric scientists, oceanographers, and modelers who were interested in reconsidering the framework in which biogeochemical cycles (ie element cycles involving the interactions of biological, geological, and chemical processes) are studied, the series consisted of 21 invited talks, a plenary, and an evening workshop. The series organizers (J Cole, E Holland, and A Finzi) attempted to include perspectives on various ways in which biogeochemical cycles are coupled (1) with each other (through biological or chemical reactions), (2) over large landscapes, (3) over various time scales, and (4) to anthropogenic factors. Contributors to this Special Issue describe key couplings (linkages) among these biogeochemical cycles that control planetary-scale ecosystem dynamics. Although space limitations allowed for inclusion of only one-third of the presentations and a limited discussion of the full richness of the topic, a complete list of the talks is available on the ESA website (http://esameetings.allenpress.com/2009).
Society reaps enormous benefits from interacting cycles when their pairing reflects the “normal” patterns of element inputs and outputs, because biological and chemical feedbacks enable organisms to tap into the entire suite of resources necessary for growth (eg water, nitrogen [N], carbon [C]), particularly those resources that are in short supply. Farmers, foresters, home gardeners, and naturalists understand that balanced inputs of carbon dioxide (CO2) from the atmosphere and nutrients and water from the soil support the productivity and diversity of managed and unmanaged ecosystems alike. However, perturbations in these cycles change the control over ecosystem processes, often leading to degraded ecosystems and limiting the services they provide. Pollution of lakes by phosphate from urban or agricultural runoff, for example, can cause algal blooms or hypoxic conditions that lead to large-scale fish kills; the deposition of N as acid rain in forests makes other important nutrients, such as calcium, more prone to loss through leaching; and emissions from the combustion of fossil fuels have changed the CO2 composition of the atmosphere and therefore Earth's climate, along with the pH of the oceans.
If the coupled-cycles framework explains many of the causes of recent global environmental degradation, it can also provide scientific guidelines for solutions. ESA recently launched a new initiative, Earth Stewardship, designed to establish a more sustainable relationship between human society and the biosphere (Power and Chapin 2009, Front Ecol Environ 7: 399). One of the most pressing needs is to restore a more natural balance among biogeochemical cycles by reducing the rates of C emissions from the burning of fossil fuels and the industrial conversion of inert atmospheric N to biologically available forms. Restoring the balance of global C and N cycles is of the utmost importance in maintaining the diversity and species composition of the planet and minimizing increases in ocean acidity.
Past fossil-fuel emissions have already committed the Earth to centuries of elevated atmospheric CO2 concentrations – concentrations that are higher than at any point during the past half-million years. This presents ecologists with challenges of societal importance. Can we envision changes in social–ecological systems, including the built environment, which could remove CO2 from the atmosphere more rapidly than it is emitted? How can the N cycle be coupled with other biogeochemical cycles to meet the world's food needs without polluting aquifers, rivers, lakes, and oceans? How can the biogeochemical cycles of unmanaged ecosystems be restored so that these systems provide the mix of benefits and services that human society requires? Answers to these questions must not only address humanity's need for food and water, but also provide the basis for a respectful and protective relationship with the biosphere that is key to our sense of cultural identity. Addressing these questions requires a broad societal commitment to the stewardship of the planet. It also requires novel applications of ecological science – including insights into the management of coupled biogeochemical cycles – to guide society toward a more sustainable future. By addressing some of these applications and insights – as well as discussing many of the changes in the coupling of biogeochemical cycles – this Special Issue strives to demonstrate how collaboration and improved communication among scientists with organismal and biogeochemical perspectives will enhance the understanding and forecasting of the planet's future environmental conditions.