Next year is shaping up as a banner year for America's art museums, thanks to a long-sought, hard-fought modification to the tax code passed by Congress last week. For one year only -- fiscal 1991 -- wealthy donors can once again give art to a museum and deduct the full market value from their taxes. Museum directors across the country are thrilled.

"It's good news, and that's the bottom line," says National Gallery of Art Director J. Carter Brown. "We don't know how good, but it's got to be better than it was."

Until 1986, it was in large part just such a tax break that fostered the explosive growth of postwar American museums. Since the 1986 Tax Reform Act, however, a provision in the federal alternative minimum tax has so penalized wealthy donors of vastly appreciated art that gifts to museums have dropped by 47 percent, according to the American Association of Museums.

"For the first time since I can remember, we are considering canceling this December's meeting of the acquisitions committee due to a lack of significant gifts to consider," says National Museum of American Art Director Elizabeth Broun. "We have received some beautiful photographs and folk art and drawings, but nothing of sufficient monetary value to require the review of our board."

"The falloff in gifts is stunning," says National Portrait Gallery Director Alan Fern. "People just don't give objects anymore. We're not crying wolf: It's been a disaster for everybody."

Though the impact on museums seems to vary from city to city, museum officials across the country agreed yesterday that it has been the low tax incentives, compounded by sky-high auction prices, that have resulted in so many major works being sold instead of donated, many of them ending up in Japan. The case of the painting "Dr. Gachet" by van Gogh is often cited as an example. On long-term loan from a private owner to the Metropolitan Museum of Art since 1984, the painting was sold at auction this year for an irresistible $82.5 million -- to a Japanese buyer.

Shows like the Los Angeles County Museum of Art's current 25th-anniversary celebration and the National Gallery's upcoming 50th-anniversary show, both of which brim with new or promised gifts, suggest that generosity has not been squelched entirely.

"For great collectors like Julian and JoAnn Ganz and others, it probably won't make a big difference," says LACMA Director Earl "Rusty" Powell III. "But it will make possible entrepreneurial situations for people with a unique tax situation, who may want to offset it with a gift.

"Nineteen ninety-one will be the interesting year," says Powell, "and has the potential of being an outstanding year for donations of works of art. Just as a result of yesterday's media coverage of the budget changes, we had two important donors call to say they'd like to discuss ideas for gifts in '91. That alone is significant: I haven't had calls like that for a long time."

"We are delighted that this error -- which we believe crept into the tax law without anyone seeing the consequences to the nation -- has been given a one-year correction," says the National Gallery's Brown, "and our hope is that it will help prove it should be made indefinite.

"I think it's an important step, and important that the Congress saw that this isn't just a windfall for the rich."

There's still a glitch for the very rich, says Brown. "What people forget is that the cap on deductions that can be taken at fair-market value is still at 30 percent of taxable income. In other words, there is no carry-over provision from year to year in the law as passed: They don't address it at all, though it's possible that the Congress would see this problem and correct it in the coming year."

As for the amount of revenue lost to the treasury because of this new provision, Brown says "it's chicken feed, compared to the gain.

"What a lot of people don't realize is that gifts in kind are what it's all about for a museum. In terms of replaceability, the museums would much rather have the picture. What can we do with the money, given today's market?" says Brown.

"I think {the tax change} will make some difference because there are people who say they won't make gifts without a deduction," says tax attorney Jerome Kurtz, former commissioner of the Internal Revenue Service.

"But it's not the good old days, when it was just as cheap to give as to sell. What would really help the museums most," he said, "is higher taxes."

He added that the new provision will probably give the appearance of helping more than it actually does, "because you'll have a bunching in 1991 on a one-year provision. But that's not normal giving." Kurtz and others suggest that such bunching, however, will probably be used to prove the importance of the change in hopes of getting it extended or made permanent.

Meanwhile, Occidental Petroleum President Bill McSweeny, who with his wife supports many art and education causes in Washington, had the most inventive take on the new possibilities. "In 1991," he says, "I might give appreciated art instead of money.

"You can't open the mail without getting a letter from your third-best friend asking for money these days," he says. "It might well be that instead of money, they'll all give appreciated art. Lots of schools have art galleries now."