Despite being a major domain of global food supply, rice-wheat cropping system is questioned for its contribution to carbon flux. Enhancing the organic carbon pool in this system is therefore necessary to reduce environmental degradation and maintain agricultural productivity. A field experiment (November 2002-March 2006) evaluated the effects of soil management practices such as tillage, crop residue, and timing of nitrogen (N) application on soil organic carbon (SOC) sequestration in the lowland of Chitwan Valley of Nepal. Rice ( Oryza sativa L.) and wheat ( Triticum aestivum L.) were grown in rotation adding 12 Mg ha -1 y -1 of field-dried residue. Mung-bean ( Vigna radiata L.) was grown as a cover crop between the wheat and the rice. Timing of N application based on leaf color chart method was compared with recommended method of N application. At the end of the experiment SOC sequestration was quantified for five depths within 50 cm of soil profile. The difference in SOC sequestration between methods of N application was not apparent. However, soils sequestered significantly higher amount of SOC in the whole profile (0-50 cm soil depth) with more pronounced effect seen at 0-15 cm soil depth under no-tillage as compared with the SOC under conventional tillage. Crop residues added to no-tillage soils outperformed other treatment interactions. It is concluded that a rice-wheat system would serve as a greater sink of organic carbon with residue application under no-tillage system than with or without residue application when compared to the conventional tillage system in this condition.
