BERLIN, June 21 (Reuters) - Germany"s liberal Free Democrats (FDP) will not join a coalition led by the ecologist Greens after an election in September, mass-selling Bild daily reported on Monday, citing party sources. The decision would make it more difficult for the Greens, who are expected to come second in the election behind Chancellor Angela Merkel"s conservatives, to try to form a government after the vote. The Greens are running about 8 points behind the conservatives and polling at about 20%. The conservatives and Greens could seek to form a coalition after the election but may find it difficult to bridge differences on whether to raise taxes to fund a transition to a carbon-neutral economy and ramp up investments in digital infrastructure and schools. The conservatives argue that tax hikes would harm the economic recovery from the coronavirus pandemic. Opinion polls show the Greens would have a majority in parliament with the FDP and centre-left Social Democrats (SPD), who are polling on about 13% and 16%, respectively. The FDP are the conservatives" preferred ruling partners but the two parties are unlikely to win enough votes for a parliamentary majority given that the conservative bloc is on 28%.