Technical Abstract: Increasing concentration of carbon dioxide in the atmosphere during the past few centuries has contributed to a now well-known phenomenon called the greenhouse effect. Soil organic carbon has historically been a source of some of this carbon dioxide, resulting from accelerated decomposition of soil organic matter following cultivation of large areas of natural landscapes, including much of the United States during the past couple of centuries. Today a large source of carbon dioxide is from the burning of fossil fuels, while agricultural land potentially represents a new sink for carbon dioxide to be fixed by plants and stored in soil organic matter. Some of the important management practices contributing to this change of soil from a source to a sink have been conservation tillage and retention of crop residues on fields. This book chapter, prepared by a scientist at the USDA Agricultural Research Service in Watkinsville Georgia, summarizes the multi-faceted changes in soil organic carbon resulting from conservation tillage adoption and retention of crop residues with no tillage. A key response of soil to these conservation practices is a more highly stratified depth distribution of soil organic carbon and nitrogen fractions. Greater surface soil organic matter and retention of surface residues are not only important for mitigating greenhouse gas emissions by sequestering carbon in soil, but also for enhancing water use by crops and preventing sediment and nutrient losses from wind and water erosion. Environmental benefits of conservation tillage and residue retention in agriculture are being realized on more than 100 million acres of cropland in the United States and there is potential for even greater benefit with further adoption.