The first round of U.S.-Soviet negotiations on a new long-term agreement on sales of American grain to the Soviet Union ended here today without producing any apparent results.

A two-sentence statement issued by the U.S. Embassy said that "talks held in Moscow were constructive. Another meeting will be scheduled." There was no official Soviet comment.

Well-informed sources suggested that the Soviets, buoyed by the prospects of an improved grain crop this year, were driving a hard bargain. Although officials described the three-day negotiations as constructive, there was no mention of any progress toward a new agreement. The two sides even failed to set the date for a new round of talks.

Members of the American negotiating team refused to meet with journalists prior to their departure from Moscow later today.

The Russians are believed to regard grain trade as providing them with some leverage on President Reagan, who suspended talks on a new agreement following the imposition of martial law in Poland in 1981 but who is seen as being under pressure from financially stricken U.S. farmers to regain access to the Soviet market.

The Soviet Union is the world's largest importer of grain while the United States is the largest producer.

Although Reagan in October offered to sell 23 million tons of grain to the Soviet Union in the current crop year ending in September, the Russians have purchased only 6.2 million tons, or the minimum amount they were committed to purchase under the existing agreement.

The sharp decline in U.S. grain exports to the Soviet Union appears to be directly linked to the overall state of Soviet-American relations. Political observers here believe that Moscow, while interested in continued access to U.S. grain, is now entertaining the idea of holding down grain purchases to stir political trouble for Reagan in the farm belt during the 1984 election campaign.

Soviet spokesmen, however, repeatedly have asserted that grain purchases are not linked to other areas of Soviet-American relations. They have argued that the United States was an "unreliable" partner and that it was using food sales as a weapon in foreign policy.

In 1980, then-president Jimmy Carter imposed a partial embargo on grain exports in retaliation for the Soviet invasion of Afghanistan, canceling contracts for 17 million tons of grain. The embargo did not affect the minimum of 6 million and the maximum of 8 million tons of grain sold to the Soviet Union annually under the 1976 agreement.

While Reagan lifted the embargo in 1981, he had resorted to export controls in an effort to block or delay certain Soviet industrial projects. Moscow's anger is reflected in the fact that while the American farmers held roughly 75 percent of the Soviet grain market prior to the embargo, this year that share has dropped to less than 20 percent.

In the meantime, the Soviets have developed new grain sources, chiefly Argentina, Canada and Western Europe, which supply most of Moscow's current grain import needs of about 35 million tons.

Moscow's position in the current situation is strengthened by better weather conditions and a new production system in agriculture this year. After four disastrous harvests starting in 1979, Moscow's grain crop is expected to total more than 200 million tons this year.

For the past four years, Soviet grain crops have fallen short of targets by 70 million to 40 million tons. Although no figures were published in recent years, U.S. Department of Agriculture specialists estimate that the 1981 crop was about 160 million tons and last year's crop only slightly higher.