Contents

17.1 Conceptual Basis for Carbon Sequestration in Soils 436

17.2 Opportunities for Carbon Sequestration in Soils 440

17.3 Impact of Tillage Management on Greenhouse

Gas Fluxes 440

17.4 GWP Analysis for Conventional and No-Tillage Maize Production in the United States 441

17.5 Long-Term GWP Effects of Changing

Tillage Practice 445

17.6 Conclusions 446

Acknowledgments 447

References 447

Carbon sequestration in soils is a land-based option to reduce the greenhouse warming potential (GWP) of the atmosphere. It has the additional benefits of improving soil quality and the sustainability of agriculture. This chapter discusses the potential for carbon sequestration in soils from a conceptual framework based on aggregation of soils as the primary process that protects organic matter from biological oxidation to form carbon dioxide. Tillage of soils breaks aggregates, and therefore leads to loss of organic carbon (OC); this process can only be prevented or reversed by stopping tillage. No-tillage crop production is already happening on a large scale in several countries, most notably the United States and Brazil. Conversion from conventional tillage (CT) to no tillage (NT) has several other GWP effects, however, in addition to increasing carbon sequestration, and these effects have been not been adequately analyzed. Consequently, it is unclear what the GWP outcome of promoting carbon sequestration through a switch to NT agriculture actually is. Given the current high level of interest in promoting carbon-trading schemes based on carbon sequestration in soil, it is surprising that neither scientists nor policymakers have recognized the need for a comprehensive evaluation of GWP effects of the change from CT to NT. This chapter extends the carbon dioxide-based analysis of West and Marland (2002) for grain production in the United States to include other greenhouse gases, showing how the timeframe of analysis can affect GWP outcome.

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