Global soil resources under stress: The future of humanity is intertwined with the future of Earth's soil resources. Soil provides for agriculture, improves water quality, and buffers greenhouse gases in the atmosphere. Yet human activities, including agricultural soil erosion, are rapidly degrading soil faster than it is naturally replenished. At this rate, human security over the next century will be severely threatened by unsustainable soil management practices. Amundson et al. review recent advances in understanding global soil resources, including how carbon stored in soil responds to anthropogenic warming. Translating this knowledge into practice is the biggest challenge remaining. Science , this issue 10.1126/science.1261071

▪ Abstract The global soil C reservoir, ∼1500 Gt of C (1 Gt = 10¹²kg of C), is dynamic on decadal time scales and is sensitive to climate and human disturbance. At present, as a result of land use, soil C is a source of atmospheric CO₂in the tropics and possibly part of a sink in northern latitudes. Here I review the processes responsible for maintaining the global soil C reservoir and what is known about how it responds to direct and indirect human perturbations.“I am fire and air; my other elements I give to baser life” —William Shakespeare, Antony and Cleopatra

Comparison of ¹⁴ C (carbon-14) in archived (pre-1963) and contemporary soils taken along an elevation gradient in the Sierra Nevada, California, demonstrates rapid (7 to 65 years) turnover for 50 to 90 percent of carbon in the upper 20 centimeters of soil (A horizon soil carbon). Carbon turnover times increased with elevation (decreasing temperature) along the Sierra transect. This trend was consistent with results from other locations, which indicates that temperature is a dominant control of soil carbon dynamics. When extrapolated to large regions, the observed relation between carbon turnover and temperature suggests that soils should act as significant sources or sinks of atmospheric carbon dioxide in response to global temperature changes.

Soils store two or three times more carbon than exists in the atmosphere as CO₂, and it is thought that the temperature sensitivity of decomposing organic matter in soil partly determines how much carbon will be transferred to the atmosphere as a result of global warming¹. Giardina and Ryan² have questioned whether turnover times of soil carbon depend on temperature, however, on the basis of experiments involving isotope analysis and laboratory incubation of soils. We believe that their conclusions are undermined by methodological factors and also by their turnover times being estimated on the assumption that soil carbon exists as a single homogeneous pool, which can mask the dynamics of a smaller, temperature-dependent soil-carbon fraction. The real issue about release of carbon from soils to the atmosphere, however, is how temperature, soil water content and other factors interact to influence decomposition of soil organic matter. And, contrary to one interpretation³ of Giardina and Ryan's results, we believe that positive feedback to global warming is still a concern.

Atmospheric carbon dioxide is the raw material for the biosphere. Therefore, changes in the carbon isotopic composition of the atmosphere will influence the terrestrial δ ¹³ C signals we interpret. However, reconstructing the atmospheric δ ¹³ C value in the geologic past has proven challenging. Land plants sample the isotopic composition of CO ₂ during photosynthesis. We use a model of carbon isotopic fractionation during C3 photosynthesis, in combination with a meta–data set (519 measurements from 176 species), to show that the δ ¹³ C value of atmospheric CO ₂ can be reconstructed from the isotopic composition of plant tissue. Over a range of pCO ₂ (198–1300 ppmv), the δ ¹³ C value of plant tissue does not vary systematically with atmospheric carbon dioxide concentration. However, environmental factors, such as water stress, can influence the δ ¹³ C value of leaf tissue. These factors explained a relatively small portion of variation in the δ ¹³ C value of plant tissue in our data set and emerged strongly only when the carbon isotopic composition of the atmosphere was held constant. Members of the Poaceae differed in average δ ¹³ C value, but we observed no other differences correlated with plant life form (herbs, trees, shrubs). In contrast, over 90% of the variation the carbon isotopic composition of plant tissue was explained by variation in the δ ¹³ C value of the atmosphere under which it was fixed. We use a subset of our data spanning a geologically reasonable range of atmospheric δ ¹³ C values (−6.4‰ to −9.6‰) and excluding C3 Poaceae to develop an equation to reconstruct the δ ¹³ C value of atmospheric CO ₂ based on plant values. Reconstructing the δ ¹³ C value of atmospheric CO ₂ in geologic time will facilitate chemostratigraphic correlation in terrestrial sediments, calibrate pCO ₂ reconstructions based on soil carbonates offer a window into the physiology of ancient plants.