Annual net carbon budget in rice soil

Crop residue or biomass is generally removed from the field as animal feed and fuel in many Asian countries. The impact of this practice on sustaining soil annual net carbon balance (ANCB) has not received much attention for cover crop-rice cropping systems in paddy soil. The influence of cover crop cultivation and biomass incorporation rates on ANCB needs to be evaluated in a rice-based cropping system. Above-ground biomass was harvested and incorporated at 3.0, 6.0 and 12.0 Mg ha(-1) (dry weight) 1 week before rice transplanting and carbon (C) inputs and losses were compared with a control treatment i.e. 0 Mg biomass ha(-1). The annual net C inputs and outputs significantly increased with the increased biomass incorporation rates. Applied organic C was lost mainly as CO2-C during cover crop cultivation, and as CH4-C and CO2-C during the rice growing season. About 81-90% of total organic C output was lost as CO2-C and 10-19% as CH4-C during the rice growing season. The net ecosystem C balance was negative (- 1402 to - 1523 kg C ha(-1)) with cover crop cultivation, but it was mostly positive (1071-2652 kg C ha(-1)) in the rice growing season depending on biomass incorporation rates. About 5.2 Mg ha(-1) (43% of total harvest) cover crop biomass needs to be recycled for sustaining soil organic C stocks in paddy field.