Estimates of global net primary production suggest that nearly all the C, N, P, and S in the Earth’s crust has spent at least some time in the biosphere, highlighting the importance of life in controlling surface geochemistry on Earth. Two axioms link the biogeochemical cycles of the important elements of life: the characteristic stoichiometry of biomass and the flow of electrons in oxidation-reduction reactions that underlie all metabolism. These principles have a long-history, beginning with Redfield’s studies of the oceans and Wakman's studies of microbiology. Coupled element cycles will constrain all human efforts to enlist ecosystem services to mitigate human impacts on the natural environment.
Results/Conclusions
Predictions of carbon uptake stimulated by excess nitrogen deposition in forest ecosystems or by additions of iron to the oceans must be compatible with estimates of how much of the triggering element is actually retained in these systems, the characteristic C/N and C/Fe ratio in primary production, and how much carbon is likely to escape the best efforts of decomposers. Organic materials also affect the local and global flux of non-essential and toxic elements. Potential metabolic pathways in specialized environments, such as the methylization of mercury in aquatic sediments, are determined by the inputs of organic carbon that maintain low redox potentials, allowing sulfate-reduction in sediments. Even for toxic elements such as lead, the annual flux through a forest landscape is determined by its adsorption to organic detritus in soils, although not in predictable stoichiometric quantities. As we invoke ecosystem services and potentially embark on geoengineering to sustain life on this planet, biogeochemists will play a critical role in recognizing the boundary conditions of what is possible.
