i,

1

43. J. Kissel and F. R. Krueger, ibid. 326, 755 (1987). 44. M. E. Lawler and D. E. Brownlee, ibid. 359, 810 (1992). 45. B. C. Clark, in Carbon in the Galaxy: Studies from Earth and Space, J. C. Tartar, S. Chang, D. J. DeFrees, Eds. (National Aeronautics and Space Administration Conference Publication, Moffett Field, CA, 1987), vol. 3061, pp. 27-36. 46. R. Zenobi, J.-M. Philippoz, P. R. Buseck, R. N. Zare, Science 246, 1026 (1989). 47. R. Zenobi et al., Geochim. Cosmochim. Acta 56, 2899 (1992). 48. S. J. Clemett, C. R. Maechling, R. N. Zare, C. M. 0. Alexander, Lunar Planet. Sci. XXIII, 233 (1992). 49. Y. Kolodny, J. F. Kerridge, I. R. Kaplan, Earth Planet. Sci. Lett. 46,149 (1980). 50. J. F. Kerridge, ibid. 64, 186 (1983). S. Chang, R. Shipp, Geochim. Cosmo51. chim. Acta 51, 2527 (1987). 52. R. Zenobi and R. N. Zare, in Advances in Multiphoton Processes and Spectroscopy, S. H. Lin,

--

p

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Ed. (World Scientific, Singapore, 1991), vol. 7, pp. 1-167. 53. R. N. Zare, C. R. Maechling, S. J. Clemett, unpublished results. 54. These identifications are based on extensive experience with standard samples and the fact that the UV laser is tuned to selectively photoionize aromatic molecules. Except for naphthalene, there are structural isomers for these assignments, and their contributions are uncertain. We do know, however, that the absorption cross section for phenanthrene and pyrene are 20 and 23 times stronger than that of their isomers anthracene and fluoranthene, respectively (52). 55. The authors thank C. M. O'D. Alexander for useful discussions and comments. This work was supported by National Aeronautics and Space Administration grants NAG 9-458 (R.N.Z.) and NAG 9-55 (R.M.W.). 13 July 1993; accepted 7 September 1993

The Effect of Changing Land Use on Soil Radiocarbon Kevin G. Harrison,* Wallace S. Broecker, Georges Bonani Most carbon budgets require greening of the terrestrial biosphere as a sink for some of the carbon dioxide produced by fossil fuel burning and deforestation. Much of this storage is thought to occur in soils, but running counter to this conclusion is the observation that cultivation has reduced the agricultural reservoir of soil humus. Radiocarbon measurements in agricultural soils lend support to this browning of agricultural lands. Moreover, the loss is from the fast cycling portion of the humus. excess

Soil contains about three times the amount of carbon that was present in the preindustrial atmosphere. This study uses soil radiocarbon measurements to explore the dynamics of soil carbon loss associated with agriculture, a significant source of atmospheric CO2. In a survey of 1100 paired soil analyses (1), agriculturally modified topsoils averaged 25% less carbon than their native counterparts. As cultivated soil now contains about 180 gigatons of carbon (GtC) (2), this loss has added 60 GtC to the atmosphere and may have contributed as much as 0.5 GtC annually during the 1980s (3). There are a variety of reasons why disturbed soil might have less carbon than its native counterpart. These include reduction in the annual input of plant residues, increased decomposition as a result of elevated soil temperature, aeration, and extra moisture (4). In addition, plowing increases surface area, which accelerates soil carbon respiration (5). Although erosion is another way in which soil carbon could be lost from the profile, it should not change the K. G. Harrison and W. S. Broecker, Department of

Geological Sciences and Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964. G. Bonani, Eidgenossiche Technische Hochschule (ETH)-Zurich, Institut fur Mittelenergiephysik, CH-8093 Zurich, Switzerland. *To whom correspondence should be addressed.

carbon concentration. Only the thickness of the topsoil would be diminished. Further, carbon losses from the soil due to erosion are much less than losses due to oxidation (4). As we have shown in a previous paper (6), based on radiocarbon data, the carbon in soil can be divided into fast and slow turnover time pools. We suggest that the carbon lost from agricultural soils must have come from the fast cycling pool. If so, then this loss should be matched by a decrease in the 14C/C ratio of bulk soil carbon. The logic is as follows: The evolution of radiocarbon in the surface of natural soil can be modeled by the assumption that 25% of the carbon resides in a slow-turnover pool with a 14C/C ratio averaging 0.63 of that for preindustrial carbon (6). Because of its slow turnover, no significant bomb 14C has entered this reservoir. The remaining 75% of the carbon resides in a fast turnover pool with a mean replacement time of 25 years. A 3:1 mix of these two end-members yields a time history that passes through the median of the available radiocarbon measurements on bulk carbon from uncultivated topsoil collected at various times and places over the globe (Fig. 1). If the 25% loss were to have come entirely from the fast cycling carbon pool, then the fast:slow proportions would be changed from a 3:1 mixture to a 2:1 mixture. Although agricultural soil has SCIENCE

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VOL. 262

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29 OCTOBER 1993

a lower radiocarbon content than native soil, the deficiency is even greater than expected for a 25% loss (Fig. 1). Rather, the best fit curve corresponds to a 1:1 mixture of slow and fast cycling carbon pools. To accomplish this would require that two-thirds of the fast cycling carbon pool be lost (that is, 50% of the total carbon). In constructing these curves, for simplicity we have assumed that the carbon loss occurred largely before the nuclear era (1958 to the present). Were the calculation to assume, instead, that half was lost after 1958, the results would change only very slightly. The shape of the best fit curve through the cultivated soil suggests that the turnover time for the fast cycling carbon pool in cultivated soil carbon is 25 years, the same as for native soil. The greater than expected 14C/C reduction for mean cultivated soil may be, in part, the result of mechanical stirring by plowing. For most soils, the 14C/C ratio decreases with depth, approaching values that are 30 to 50% lower than the prenuclear atmospheric ratio at the base of the profile. We attribute this drop to an everdecreasing fractional contribution of the fast turnover carbon with depth. But, because plowing homogenizes only the upper 20 cm of soil, its impact would not be expected to be large. Data on carbon content and 14C/C ratio on a native soil from New Zealand (7) are

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Fig. 1. Plot of radiocarbon versus time for the bulk carbon in topsoils (8). The natural soils (open circles) have higher radiocarbon values than the cultivated soils (solid triangles). The thick solid lines represent new vegetation, a fast (25-year turnover time) carbon pool's, and a slow (3700-year turnover time) carbon pool's responses to atmospheric bomb radiocarbon. The thin lines designate mixtures of fast and slow cycling carbon. A 75% fast and 25% slow mixture provides the best fit for the natural soils, but a 50% fast and 50% slow mixture provides the best fit for the cultivated soils. 725

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Fig. 2. Hypothetical evolution of 0 native New Zealand soil carbon 10 and radiocarbon subject to changing land use. The top fig30 ures illustrate the native radiocarbon ratios and carbon inventories, both of which decrease with 5 0.1 0.2 0.3 depth (6). Also shown is the parE titioning between fast turnover 1' 10 > 1 2 20 (unshaded region) and slow turnIAfter over (shaded region) carbon ' 30 mixing by plowing pools. The fast pool has a 14C/C o40 ratio of 1.26 and the slow pool a ac 0.3 0.2 0.1 ratio of 0.63. The middle figures 0 After 25% show how a farmer's plow would 10 oxidative homogenize the upper 20 cm, 20 loss thus decreasing the observed 30 40 14C/C ratio and carbon inventory 4 for the surface soil. The bottom 5 figures show how the 14C/C ratio 0.1 0.2 0.3 and carbon inventory change with Carbcon inventory (glcm2) a 25% loss of carbon, all from the fast-turnover pool.

consistent with the hypothesis that a combination of plowing and oxidation of the fast cycling carbon pool are the cause of the downward shift in 14C/C ratios observed for agricultural soil. To test this hypothesis, we considered what would happen if a native New Zealand soil was mixed to a depth of 20 cm and 25% of the carbon was oxidized (Fig. 2). Through mixing with underlying material, the 14C/C ratio for the upper 10 cm of soil was reduced from a native value of 1.08 to 1.02 and the soil carbon inventory in this layer was reduced from 0.26 to 0.22 g/cm2. We assumed that the increased oxidation took place in the fast cycling pool with an e-folding time of 25 years, eventually removing 25% of the soil's carbon. As a result, after a few decades, the 14C/C ratio in the upper 10 cm of the soil dropped to 0.94 and the bulk carbon to 0.16 g/cm2. The predicted 14C/C ratio of 0.94 agrees with the observed average value for cultivated soil collected in 1975. In summary, the observation that the 14C/C ratios for agricultural soil are lower than those for native soils is consistent with a reduction in the amount of humus stored in these soils. Part, but not all, of the radiocarbon reduction can be attributed to preferential oxidation of humus in the fast cycling pool relative to that in the slow cycling pool. These observations suggest that as much as 60 GtC (10 years of fossil fuel CO2 production at the current rates) could be sequestered if agricultural soil across the globe could be engineered back to its original carbon content. REFERENCES AND NOTES 1. W. M. Post and L. K. Mann, in Soils and the Greenhouse Effect, A. F. Bowman, Ed. (Wiley, New

York, 1990), pp. 401-407. 2. W. H. Schlesinger,

726

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4. W. H. Schlesinger, in The Changing Carbon Cycle-A Global Analysis (Springer-Verlag, New York, 1986), pp. 194-200. 5. J. J. Schoenau, thesis, University of Saskatchewan

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Global Change (Academic Press, New York, 1991), p. 135. 3. We estimated annual carbon fluxes using estimates of cultivated land area with time made by J. F. Richards [in The Earth Transformed by Human Action: Global and Regional Changes in the Biosphere over the Past 300 Years, B. L. Turner II et al., Eds. (Cambridge Univ. Press, New York, 1990), pp. 163-178] and Schlesinger's (2) estimate that agricultural soil contains 180 GtC.

(1988). 6. K. G. Harrison, W. S. Broecker, G. Bonani, Global Biogeochem. Cycles 7, 69 (1993). 7. B. J. O'Brien, Radiocarbon 28, 358 (1986). 8. The data for uncultivated surface soils were listed in (6). New data were obtained for uncultivated surface soils in Lodi, Italy (collected in 1984, 14C = 109% modern), Saskatoon, Saskatchewan, Canada (collected in 1985, 14C = 122% modern), Konza Prairie, KS (collected in 1991, 14C = 115% modern), and Durham, NC (collected in 1991, 14C = 112% modern). The data for cultivated surface soils was obtained from: D. W. Anderson and E. A. Paul, Soil Sci. Soc. Am. J. 48, 298 (1984); Y. A. Martel and E. A. Paul, Can. J. Soil Sci. 54, 419 (1974); Soil Sci. Soc. Am. Proc. 38, 501 (1974); H. W. Scharpenseel, P. Becker-Heidmann, H. U. Neue, K. Tsutsuke, Sci. Total Environ. 81, 99 (1989); S. E. Trumbore, G. Bonani, W. Wolfi, in Soils and the Greenhouse Effect, A. F. Bouwman, Ed. (Wiley, New York, 1990), pp. 407-414; (7). Values for cultivated surface soils and published for the first time in this paper are from: Lodi, Italy (collected in 1984, 14C = 103% modern), Konza Prairie, KS (collected in 1991, 14C = 101% modern), and Saskatoon, Saskatchewan, Canada (collected in 1985,14C = 103% modern). 9. This research was supported by a grant from the Department of Energy and a National Aeronautics and Space Administration Globa Change Fellowship. We thank two anonymous reviewers, whose critiques led to a greatly improved manuscript, and B. Zambella, who lent a discerning eye, careful hand, and timely resources. LDEO contribution S128. 10 May 1993; accepted 8 September 1993

Crustal Thickness on the Mid-Atlantic Ridge: Bull's-Eye Gravity Anomalies and Focused Accretion Maya Tolstoy, Alistair J. Harding, John A. Orcutt Spreading segments of the Mid-Atlantic Ridge show negative bull's-eye anomalies in the mantle Bouguer gravity field. Seismic refraction results from 330S indicate that these anomalies can be accounted for by variations in crustal thickness along a segment. The crust is thicker in the center and thinner at the end of the spreading segment, and these changes are attributable to variations in the thickness of layer 3. The results show that accretion is focused at a slow-spreading ridge, that axial valley depth reflects the thickness of the underlying crust, and that along-axis density variations should be considered in the interpretation of gravity data.

The source of bull's-eye-shaped mantle Bouguer anomalies (MBAs) on the MidAtlantic Ridge (MAR) has been debated since they were first observed in 1988 (1). The MBA represents a simple correction of the observed gravity field for the effects of topography at both the sea floor and the Moho, assuming a constant crustal thickness (6 km) and constant densities [1030 kg/m3 for seawater, 2730 kg/M3 for crust, and 3330 kg/M3 for mantle (2)]. Three Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093.

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VOL. 262 * 29 OCTOBER 1993

possibilities for these anomalies have been proposed (1, 3-5): (i) that they provide a window into the dynamics of the underlying mantle and are evidence of central upwelling plumes; (ii) that they may be attributable to along-axis crustal thickness variations; or (iii) a combination of the two. A seismic measurement can distin-

guish between these possibilities by providing an independent estimate of crustal thickness. Any of these explanations would indicate that magmatic accretion is focused (3) on the slow-spreading MAR. In focused accretion, the middles of segments are

Downloaded from www.sciencemag.org on December 7, 2009

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The Effect of Changing Land Use

Jul 13, 1993 - (3). There are a variety of reasons why disturbed soil might have less carbon than its native .... B.C. Clark, in Carbon in the Galaxy: Studies from.

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