Ex situ nitrification followed by denitrification inside the landfill has been recommended to remove ammonia from leachate. The effects of increasing nitrate load and decreasing organic carbon content in the injected leachate on the denitrifying capacity of municipal solid waste (MSW) were investigated. Results showed that MSW possesses a high denitrification capacity. Nitrate reduction could be initiated within 48 h after the first addition of nitrate. Nitrate reduction rate increased with the increasing nitrate loading concentration. When the nitrate loading concentration was increased to 850 mg L(-1), nitrate reduction rate reached up to 35 mg L(-1) h(-1). Nitrite accumulation could be found after the addition of nitrate in each test. However, the maximum nitrite accumulation efficiency declined with increased nitrate load. Organic carbon played an important role in the reduction of nitrate, and both endogenous and exogenous organic materials could act as electron donors. (C) 2009 Elsevier Ltd. All rights reserved.