The Commerce Department, buoyed by record high U.S. exports in March, appears increasingly confident of a further substantial reduction this year in the U.S. merchandise trade deficit.

Commerce Secretary Robert Mosbacher said Thursday he now expects that the deficit will fall below $100 billion this year. In 1989, it totaled $109 billion.In March, Commerce reported Thursday, the U.S. deficit rose to $8.4 billion, after adjusting for seasonal factors. But, Mr. Mosbacher noted, the deficit in the first three months this year was down to $23.9 billion, the lowest quarterly deficit in more than six years.

Mr. Mosbacher's comments seemed more optimistic than the department's trade estimates of only a few weeks ago.

In its annual trade outlook analysis, the department said that while the deficit was expected to improve further this year, there are "greater uncertainties this year than usual" about U.S. and foreign economic growth and exchange rates.

A declining trade deficit, propelled by rising exports, would be especially welcome this year, private economists said.

The exports, said Howard Lewis, a National Association of Manufacturers (NAM) vice president, would help spur the U.S. economy and thereby help mitigate the U.S. budget deficit problem. A falling trade deficit, he added, might help smooth the way in Congress next year for the trade agreements the United States is negotiating at the General Agreement on Tariffs and Trade in Geneva.

But he and other private analysts appeared less optimistic than Mr. Mosbacher about the U.S. trade balance. Willard Workman, the U.S. Chamber of Commerce's director of international policy, projects a trade deficit this year of about $104 billion.

The unusually low $6.1 billion deficit in February, he suggested, was "an aberration," due partly to higher than average aircraft exports.

C. Fred Bergsten, director of the Institute for International Economics, a Washington, D.C.-based research group, said that the U.S. deficit is ''basically stuck at around the $100 billion level." For it to decline further, he said, requires a further depreciation of the dollar, especially against the Japanese yen and the Taiwan dollar.

The first quarter U.S. deficit was lower than expected, NAM's Mr. Lewis said. But he added: "I wouldn't bet the farm that (this trend) will continue." Imports, he noted, were at a near-record level in March. NAM has projected a deficit of between $105 billion and $110 billion this year.

U.S. exports in March, the Commerce Department said, soared to $35.7 billion, exceeding theprevious monthly high in March 1989 by $2.6 billion. Adjusted for seasonal factors, Commerce said, the export total was $33.2 billion, still $2 billion higher than any prior month.

Exports of two broad product categories - foods, feeds and beverages and automotive products - were at record levels, while shipments of other capital and consumer goods were at near-record rates, on a seasonally adjusted basis, Commerce reported.

Compared with February, exports, unadjusted for seasonal variations, were substantially higher across a broad range of items. Especially big advances came in office equipment, industrial and electrical machinery, automotive parts and chemicals.

Imports, at $41.9 billion, exceeded any previous month except last October. Adjusted for seasonal factors, imports were $41.7 billion, down only about $200 million from the October record.

New highs, taking seasonal factors into account, were scored in automotive products and foods, feeds and beverages, Commerce said. Among the largest gainers, compared with February, were cars and car parts, telecommunications gear, electrical and industrial machinery.

The oil import bill, at $4.7 billion, was only barely higher than in February, even though there were three more days in March. Imports averaged 8.4 million barrels a day in March, the lowest since December, while prices averaged $18.18 a barrel, also the lowest since December.

Steel imports, which totaled 1.12 million net tons in March, were about the same as in February, but down almost 100,000 tons from March 1989. Imports represented only 13.5 percent of U.S. consumption in March, the lowest level since the mid-1970s, the American Iron and Steel Institute indicated.

The United States in March, Commerce said, chalked up another big trade surplus with the 12-nation European Community - slightly over $1 billion. But the U.S. deficit with Japan rose again to $3.6 billion, largely on a surge in Japanese car shipments.

Japanese carmakers apparently stepped up their shipments to meet the March 31 deadline in the annual export quota the Japanese government imposes on cars

sent to the United States.

In the first three months of this year, however, the U.S. trade deficit with Japan fell to $9.6 billion, almost $3 billion less than a year earlier.

The U.S. trade balance with Canada again improved in March when the deficit fell to $90 million. In the first quarter of this year, the deficit with Canada totaled $952 million, down more than two-thirds from a year earlier.