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Estimated Increase in Atmospheric CO 2 due to Worldwide Decreases in Soil Organic Matter

Estimated Increase in Atmospheric CO 2 due to Worldwide Decreases in Soil Organic Matter. R.W. Mullen, W.E. Thomason, and W.R. Raun. Introduction. Atmospheric CO 2 has increased over the last 150 years from 260 to 340 mg kg -1 (360) Expected to rise 1.5 to 2.0 ppm per year (Wittwer, 1985)

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Estimated Increase in Atmospheric CO 2 due to Worldwide Decreases in Soil Organic Matter

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  1. Estimated Increase in Atmospheric CO2 due to Worldwide Decreases in Soil Organic Matter R.W. Mullen, W.E. Thomason, and W.R. Raun

  2. Introduction • Atmospheric CO2 has increased over the last 150 years from 260 to 340 mg kg-1 (360) • Expected to rise 1.5 to 2.0 ppm per year (Wittwer, 1985) • Responsible for 0.5 °C global temp increase • Increasing atmospheric CO2 is due to industrial burning of fossil fuels and changing land use (deforestation and cultivation) • Benefits associated with increased atmospheric CO2 (increased water use efficiency, nitrogen use efficiency and production in many crops)

  3. Introduction • The amount of carbon released by industrial processes and changing land use was estimated to be 5.0 x 1012 and 2.0 x 1012 kg C yr -1, respectively • Carbon can be sequestered by the crop-root system and redistributed deeper into the soil profile, making it less likely to be converted back to CO2

  4. Introduction • Conventional tillage practices (moldboard plow, disk harrow, chisel plow, etc.) can release carbon as CO2 via the accelerated decomposition of soil organic matter • Organic Matter decomposition = global warming

  5. Objective • The objective of this work was to derive a simple estimate of CO2 in the atmosphere that could be attributed to tillage and decomposition of soil organic matter.

  6. Discussion • Soil organic matter has declined in agricultural soils largely due to cultivation (Boman et al., 1996 and Reicosky, 1994). • Estimates of soil organic matter loss since initial cultivation range from as low as 20% (Schlesinger, 1986) to as high as 54% (Smith et al., 1997). • Native prairie soils in the Central Great Plains contained 4% soil organic matter in the 1800s, and after more than 150 years of cultivation that number is now less than 1% (Boman et al., 1996).

  7. Discussion • For this work, a 3% loss in organic matter from arable soils worldwide was assumed.

  8. TABLE 1. Components used for calculating increased atmospheric CO2 due to worldwide decreases in soil organic matter, assuming a decrease from 4% to 1% over the past 150 years on worldwide arable land. Weight of 1 hectare of soil to a depth of 15 cm (soil bulk density of 1.49 Mg/m3) 2 235 000 kg ha-1 10 000 m2 * 1.49 Mg m-3 * 0.15 m Organic carbon lost 1.47% Organic carbon = (organic matter – 0.35)/1.8 (Ranney, 1969) Carbon lost from organic matter per hectare 32 845.5 kg ha-1 2 235 000 kg ha-1 * 0.0147 Arable land in the world 1 381 917 000 ha www.fao.org, 1996 Total carbon lost from all arable land in the world 4.55 x 1013 kg 32 845.5 kg ha-1 * 1 381 917 000 ha 60% of carbon lost from organic matter converted to CO2 2.73 x 1013 kg 4.55 x 1013 kg * 0.60 (Brady and Weil, 1996) Total CO2 lost to the atmosphere 1.00 x 1014 kg 2.73 x 1013 kg * 3.67 ((44 g/mol CO2)/(12 g/mol C)) Mass of Earth's Atmosphere 5.00 x 1018 kg Wild, 1993 Change in atmospheric CO2 0.008% 80 mg kg-1 (Lal et al., 1997) change in CO2/10 000 Increase in atmospheric CO2 4.00 x 1014 kg 5.00 x 1018 kg * 0.00008 Change in atmospheric CO2 due to organic matter decay 25.03% 1.00 x 1014 kg/400 x 1014 kg Increase in atmospheric CO2 due to 3% loss of organic matter worldwide 20.03 mg kg-1 80 mg kg-1 * 0.2503

  9. Carbon lost from organic matter 48052.5 kg 6.64 x 1013 kg (Schlesinger, 1984 and 1995) / 1 318 917 000 ha Organic carbon lost 2.15% 48052.5 kg / 2 235 000 000 Organic carbon matter lost 4.22% (0.0215 * 180) + 0.35 (Ranney, 1969) Total carbon lost from world 6.64 x 1013 kg Schlesinger, 1984 and 1995: 3.6 x 1013 kg + (38 yrs * 8.00 x 1011 kg) 60% of carbon lost from organic matter converted to CO2 3.984 x 1013 kg 6.64 x 1013 kg * 0.60 (Brady and Weil, 1996) Total CO2 lost to atmosphere 1.462 x 1014 kg 3.984 x 1013 kg * 3.67 Change in atmospheric CO2 due to organic matter decay 36.55% 1.462 x 1014 kg/4.00 x 1014 kg Increase in atmospheric CO2 due to 3% loss of organic matter worldwide 29.24 mg kg-1 80 mg kg-1 * 0.3655 TABLE 2. Components used for the calculating increased atmospheric CO2 due to worldwide decreases in soil organic matter, using Schlesinger (1984 and 1995) data.

  10. Conclusion • The continuous tillage of arable land worldwide is likely responsible for 6 to 36% of the increase in atmospheric CO2 due to decreased soil organic matter.

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