Low-carbon farms can raise food output - FAO

* Low-carbon farms can boost food yields

* Need funding from rich nations

BARCELONA, Spain, Nov 5 (Reuters) - Low-carbon farming can both curb climate change and boost food output in developing nations and so must be rewarded under a global climate deal due in December, the U.N.'s food agency said on Thursday.

Steps to cut carbon emissions on farms in developing countries could also boost yields where food is shortest, the Food and Agriculture Organisation said in a report published on Thursday.

More than 1 billion people are undernourished now and the world will have to feed an additional 3 billion by 2050, many in areas expected to be worst afflicted by climate change, experts say.

Certain farm practices can tackle both problems, for example conserving over-grazed pastures and caring for soils, but they involve up-front costs.

"A key part of the problem is a lack of financing," said Leslie Lipper, FAO economist and co-author of its report "Food Security and Agricultural Mitigation in Developing Countries", published on the sidelines of U.N. climate talks in Barcelona.

"If adopted by farmers, many of these practices make them better off, but in the short run they may face reduced income," Lipper said, using the example of removing cattle to allow grasslands to recover.

Agriculture has barely been mentioned in the Nov. 2-6 Barcelona talks -- the final preparatory session before a meeting in Copenhagen in December meant to agree a global climate deal to replace or extend the Kyoto Protocol.

Farms accounts for 10-12 percent of global greenhouse gas emissions directly, not including their contribution to deforestation, according to a U.N. panel of climate scientists.

An FAO study this year put the extra farm investment needed to boost food yields at $210 billion between now and 2050.

Some of the funding for low-carbon practices could come from carbon markets, whereby rich nations pay for cuts in developing countries to offset against their own emissions.

Low-carbon farming in developing nations could raise up to $30 billion annually through such carbon finance, Thursday's study said.

One difficulty is the challenge of measuring the carbon cuts, such as the extra carbon locked in the soil as a result of practices such as tilling the soil less and applying more organic fertilisers such as manure and crop waste.

The cost of soil measurement means carbon markets may only work for the most effective carbon-cutting systems, and the rest need public finance. Finding adequate funds to cut carbon emissions in developing countries has been a long-running stumbling block at the two-year U.N. talks.

(Editing by Tim Pearce)