Responses and feedbacks of coupled biogeochemical cycles to climate change: examples from terrestrial ecosystems

© The Ecological Society of America www.frontiersinecology.org I an era of extensive environmental change, human activity – particularly the harvest of resources for food, fiber, and fuel – is substantially altering Earth’s climate and its element cycles (Vitousek et al. 1997; Figure 1). The need for energy to support economic growth has increased atmospheric carbon dioxide (CO2) concentrations by nearly 40% since the beginning of the Industrial Revolution (Canadell et al. 2007). The development of the Haber–Bosch process used in the manufacture of nitrogen (N) fertilizers has more than doubled the quantity of reactive N entering the terrestrial biosphere (Galloway et al. 2008). Mining of phosphorus (P) for fertilizers has redistributed P across Earth’s surface, substantially depleting phosphate mineral deposits (Gilbert 2009) and, in combination with N, leading to the eutrophication of aquatic ecosystems (Conley et al. 2009). Individually and collectively, changes in these element cycles are reorganizing biological communities, with important implications for the health of ecosystems and Earth’s climate in the 21st century. The metabolic pathways that enable life on Earth couple biogeochemical cycles to one another (Falkowski et al. 2008). Accordingly, a change in one element cycle invariably entrains a change in one or more different element cycles (Figure 2). This entrainment raises serious concerns about human-driven increases in atmospheric CO2 concentrations that cause climate change and the biogeochemical consequences of vast quantities of N and P now flowing across the terrestrial landscape. To address the impacts of human activities on coupled biogeochemical cycles (herein CBCs), we first discuss the basis by which organisms couple biogeochemical cycles to one another. We then focus on the constraints CBCs impose on terrestrial productivity and the consequences these constraints may have for climate change through the 21st century.