Flying and swimming animals cruise at a Strouhal number tuned for high-power efficiency

In the Northern Hemisphere, ozone levels in the troposphere have increased by 35 per cent over the past century, with detrimental impacts on forest and agricultural productivity, even when forest productivity has been stimulated by increased carbon dioxide levels. In addition to reducing productivity, increased tropospheric ozone levels could alter terrestrial carbon cycling by lowering the quantity and quality of carbon inputs to soils. However, the influence of elevated ozone levels on soil carbon formation and decomposition are unknown. Here we examine the effects of elevated ozone levels on the formation rates of total and decay-resistant acid-insoluble soil carbon under conditions of elevated carbon dioxide levels in experimental aspen (Populus tremuloides) stands and mixed aspen–birch (Betula papyrifera) stands. With ambient concentrations of ozone and carbon dioxide both raised by 50 per cent, we find that the formation rates of total and acid-insoluble soil carbon are reduced by 50 per cent relative to the amounts entering the soil when the forests were exposed to increased carbon dioxide alone. Our results suggest that, in a world with elevated atmospheric carbon dioxide concentrations, global-scale reductions in plant productivity due to elevated ozone levels will also lower soil carbon formation rates significantly. Large areas of the Earth are exposed to concentrations of tropospheric ozone (O3) that exceed levels known to be toxic to plants . In addition to reducing plant growth, exposure to elevated O3 can also alter plant tissue chemistry and reduce allocation of carbon to roots and root exudates. Whereas the effects of O3 on these aspects of plant biology have been widely investigated in chamber studies, examination of these effects on below-ground carbon cycling in intact forests only became possible in 1997 with the establishment of the FACTS-II (forest–atmosphere carbon transfer and storage) FACE (free-air carbon dioxide enrichment) experiment in Rhinelander, Wisconsin, USA. The long-term FACE experiment in Rhinelander examines how plant–plant and plant–microbe interactions may alter ecosystem responses to elevated O3 and carbon dioxide (CO2) through four treatments: control, elevated CO2, elevated O3 and elevated O3 þ CO2. In plots where O3 and CO2 are elevated, concentrations were maintained at ,150% of ambient levels. To examine the effects of atmospheric trace gases on both ecological interactions and on whole-ecosystem carbon cycling, each plot is split to include a pure aspen forest, a mixed aspen–birch forest and a mixed aspen–maple forest. These species were chosen because they are among the most widely distributed trees in northern temperate forests. Here we compare soil carbon formation in aspen and aspen– birch subplots under elevated CO2 and elevated O3 þ CO2 (three plots each) to understand how exposure to O3 under elevated CO2 alters soil carbon formation. We used CO2 derived from fossil fuel with its highly depleted C signature to fumigate plant canopies in the elevated CO2 and elevated O3 þ CO2 plots. Leaf and root carbon inputs in the elevated CO2 and elevated O3 þ CO2 plots had a dC signature of 241.6 ^ 0.4‰ (mean ^ s.e.) in contrast to leaf and root inputs of 227.6 ^ 0.3‰ in the control plots. The dC signature of soil carbon was 226.7 ^ 0.2‰ before fumigation and