R. F. Follett

Objectives Basic Processes The Greenhouse Process Global Trends in Greenhouse Gas Emissions The Role of Agriculture in U.S. Emissions of Three GHGs The SOC Pool in U.S. Soils and SOC Loss from Cultivation Processes in Governing Emissions from the Pedosphere Plant Action Soil Processes Soil Quality Strategies for Mitigating Emissions from Cropland U.S. Cropland Sustainable Management Studies Soil Erosion Management Land Conversion and Restoration Conversion of Marginal Land Restoration of Degraded Soils Biofuels for Offsetting Fossil Fuel Intensification of Prime Agricultural Land Conservation Tillage and Residue Management Irrigation Water Management Improved Cropping Systems The Carbon Sequestration Potential of Arable Land U.S. Cropland's Overall Potential to Mitigate the Greenhouse Effect Techniques for Sequestration Rates of SOC Sequestration Possible Implementation Obstacles Required Action Conclusions - The Win-Win Strategy Agricultural Profits from Environmental Improvements SOC's Monetary Value SOC's Environmental Value Global Potential Appendix 1: Definitions Appendix 2: Researchable Topics

World soils have been a source of atmospheric carbon dioxide since the dawn of settled agriculture, which began about 10 millennia ago. Most agricultural soils have lost 30% to 75% of their antecedent soil organic carbon (SOC) pool or 30 to 40 t C ha−1. The magnitude of loss is often more in soils prone to accelerated erosion and other degradative processes. On a global scale, CO2-C emissions since 1850 are estimated at 270 ± 30 giga ton (billion ton or Gt) from fossil fuel combustion compared with 78 ± 12 Gt from soils. Consequently, the SOC pool in agricultural soils is much lower than their potential capacity. Furthermore, depletion of the SOC pool also leads to degradation in soil quality and declining agronomic/biomass productivity. Therefore, conversion to restorative land uses (e.g., afforestation, improved pastures) and adoption of recommended management practices (RMP) can enhance SOC and improve soil quality. Important RMP for enhancing SOC include conservation tillage, mulch farming, cover crops, integrated nutrient management including use of manure and compost, and agroforestry. Restoration of degraded/desertified soils and ecosystems is an important strategy. The rate of SOC sequestration, ranging from 100 to 1000 kg ha−1 year−1, depends on climate, soil type, and site-specific management. Total potential of SOC sequestration in the United States of 144 to 432 Mt year−1 (288 Mt year−1) comprises 45 to 98 Mt in cropland, 13 to 70 Mt in grazing land, and 25 to 102 Mt in forestland. The global potential of SOC sequestration is estimated at 0.6 to 1.2 Gt C year−1, comprising 0.4 to 0.8 Gt C year−1 through adoption of RMP on cropland (1350 Mha), and 0.01 to 0.03 Gt C year−1 on irrigated soils (275 Mha), and 0.01 to 0.3 Gt C year−1 through improvements of rangelands and grasslands (3700 Mha). In addition, there is a large potential of C sequestration in biomass in forest plantations, short rotation woody perennials, and so on. The attendant improvement in soil quality with increase in SOC pool size has a strong positive impact on agronomic productivity and world food security. An increase in the SOC pool within the root zone by 1 t C ha−1 year−1 can enhance food production in developing countries by 30 to 50 Mt year−1 including 24 to 40 Mt year−1 of cereal and legumes, and 6 to 10 Mt year−1 of roots and tubers. Despite the enormous challenge of SOC sequestration, especially in regions of warm and arid climates and predominantly resource-poor farmers, it is a truly a win-win strategy. While improving ecosystem services and ensuring sustainable use of soil resources, SOC sequestration also mitigates global warming by offsetting fossil fuel emissions and improving water quality by reducing nonpoint source pollution.

Science-based policy is essential for guiding an environmentally sustainable approach to cellulosic biofuels.

The ability to inventory soil C on landscapes is limited by the ability to rapidly measure soil C. Diffuse reflectance spectroscopic analysis in the near‐infrared (NIR, 400–2500 nm) and mid‐infrared (MIR, 2500–25000 nm) regions provides means for measurement of soil C. To assess the utility of spectroscopy for soil C analysis, we compared the ability to obtain information from these spectral regions to quantify total, organic, and inorganic C in samples representing 14 soil series collected over a large region in the west central United States. The soils temperature regimes ranged from thermic to frigid and the soil moisture regimes from udic to aridic. The soils ranged considerably in organic (0.23–98 g C kg −1 ) and inorganic C content (0.0–65.4 g CO 3 ‐C kg −1 ). These soil samples were analyzed with and without an acid treatment for removal of CO 3 Both spectral regions contained substantial information on organic and inorganic C in soils studied and MIR analysis substantially outperformed NIR. The superior performance of the MIR region likely reflects higher quality of information for soil C in this region. The spectral signature of inorganic C was very strong relative to soil organic C. The presence of CO 3 reduced ability to quantify organic C using MIR as indicated by improved ability to measure organic C in acidified soil samples. The ability of MIR spectroscopy to quantify C in diverse soils collected over a large geographic region indicated that regional calibrations are feasible.