When considering the impact of industrial processes on the environment, it is far easier to grasp direct examples-like huge piles of mining waste or runoff streaming into a river-than indirect influences far away from the source. By analyzing the isotopic signature of sediment cores from several pristine lakes in the Northern Hemisphere, Holtgrieve et al. (p. 1545; see the Perspective by Elser) demonstrate that human activity began adding substantial quantities of reactive nitrogen by the late 19th century, before fertilizer production, a known source of reactive nitrogen, became prevalent. These elevated levels may have been from burning coal residues and, although low, as com pared with direct runoff, may have been substantial enough with influence biogeochemical cycles and rates of primary productivity far outside the direct footprint of early industrial activities.
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