U.S. farmers can play a significant role in curbing greenhouse gas emissions, but more research and greater incentives are needed to make it cost-effective, according to studies released on Thursday.

The Pew Center on Global Climate Change said in two reports that farmers could reduce total U.S. greenhouse gas emissions by 5 to 14 percent by storing more carbon in agriculture soils and reducing nitrogen oxide and methane emissions.

It also estimated that research and development on biofuels could one day trim emissions up to 24 percent by substituting renewable fuels for fossil fuels.

Pew said U.S. lawmakers will need to increase research and boost funding in upcoming legislation to show farmers that changing their business practices is cost-effective and that other measures -- such as using their land to remove and store carbon or growing biomass for energy use -- can be profitable.

"Farmers will need economic incentives to change their management if there are increased costs of adopting more greenhouse gas-friendly practices," said Keith Paustian, a professor at Colorado State University who co-authored one of the studies.

"If new practices become more profitable in the long term they may need incentives to make the initial changes," he said.

U.S. agriculture is responsible for about 8 percent of total U.S. greenhouse gas emissions, Pew estimated.

The United States, the world's leading emitter of greenhouse gases, released about 7.075 billion metric tonnes of carbon dioxide equivalent last year, according to the Environmental Protection Agency. U.S. greenhouse gas emissions, which include carbon dioxide, methane and nitrous oxide, have risen 15.8 percent from 1990 to 2004.

Pew said carbon sequestration would be more attractive for lower carbon prices -- fewer than $50 per tonne of carbon and $13 per tonne of CO2 -- while biofuel and afforestation become more attractive at higher prices.

In its reports, Pew said:

-- changes to farming practices and financial incentives could result in up to 70 million tonnes of carbon to be stored in U.S. agricultural land annually, a process known as carbon sequestration.

--boosting yields of biomass crops and reducing the cost of technology used to convert them into fuel could one day supply up to 19 percent of total U.S. energy consumption, cutting CO2 emissions by up to 1.71 billion tonnes per year.

--converting 115 million acres of marginal agricultural lands to forests could capture an additional 270 million tonnes of carbon per year for 100 years, or about 20 percent of current emissions from CO2 emitted by fossil fuels.