In this Issue, students explore large-scale anthropogenic changes to the nitrogen cycle by examining studies that document anthropogenic inputs of nitrogen globally, nitrogen saturation of temperate forests, the export of nitrogen from rivers to the ocean, and the ecology of Pfiesteria in the Chesapeake Bay.
Human activity has had a major impact on global nitrogen cycling. Synthetic fixation of nitrogen gas for fertilizer production, fossil fuel burning, and cultivation of leguminous crops has approximately doubled the annual transfer of unavailable N2 to biologically available forms (see first data set). Not surprisingly the impacts of this additional N are great and include higher emissions of nitrous oxide, nitric oxide, and ammonia to the atmosphere, changes to global C cycling, nitrogen saturation in forests, eutrophication of water bodies, and acidification of lakes.
The consequences of some of these alterations to N cycling are already evident. Nitrous oxide in the atmosphere is significantly contributing to global warming. Some forests are beyond the fertilization stage of N additions and are exhibiting the symptoms of N saturation. Nitric acid contributes to acidification in poorly buffered lakes. Outcomes of nutrient loading to fresh and marine waters is arguably the most serious aquatic pollution problem that we have - anoxia and accompanying "fish" kills, tremendous growth of opportunistic species responsive to N and P, and loss of biodiversity have been all too common for decades.
The first data set featured in this Issue document the scale of anthropogenic changes to N cycling globally and is easily accessible to students. The second quantifies how N loadings to watersheds has greatly increased river and marine nutrient concentrations. The third data set focuses on "the cell from hell" and its effects - massive fish kills in the Chesapeake Bay as a result of overflow of hog farm cesspools and blooms of the dinoflagellate Pfiesteria piscicada.
The papers also typify ecosystem-scale studies. The perspectives are nitrogen and carbon cycling; partitioning of N into atmospheric, terrestrial, and aquatic fractions; systems analysis; input/output budgets; nutrient limitation; and changes in species composition with nutrient addition (bottom-up controls).