The Soviet Union and China tackled the major issues dividing them in the second round of high-level talks between the two Communist giants, and trade negotiators for the two countries have reached an accord that will boost the level of their commerce by 150 percent in 1983, sources from both sides disclosed today.

In keeping with the low-key approach, the conclusion of the second round was not announced. The government news agency Tass reported tonight, however, that Chinese negotiator Qian Qichen and his delegation had left by air for Tashkent, where they will spend four days as guests of Soviet Uzbekistan "to familiarize themselves with life in that Soviet central Asian republic."

Sources on both sides said that the negotiations were held in a "warm and friendly" atmosphere that also had marked the first round of talks in Peking last October. They added that all aspects of relations were discussed including China's list of "impediments" to an improved bilateral relationship.

The "impediments" include the Soviet military involvement in Afghanistan, Moscow's support for the Vietnamese occupation of Cambodia, and the presence of Soviet forces in Mongolia as well as along the Sino-Soviet border.

Although the negotiations seemed to have produced no major political breakthrough, it is apparent that the quality of Sino-Soviet relations is changing slowly. The most significant evidence to this effect is the newly concluded trade agreement for 1983 signed here Monday by China's Deputy Foreign Trade Minister Jia Shi. According to the sources, the agreement will expand by more than 150 percent the volume of trade over the 1982 figure.

The bulk of new Chinese exports to the Soviet Union will involve textiles and food. The Russians in turn are to provide China with timber and steel. Engineering and technical items sought by the Chinese were excluded from the pact, presumably awaiting an overall improvement of relations.

The decision to import large quantities of Chinese textile products was seen by observers here as a political move by Moscow. It comes in the wake of a Sino-American trade dispute over U.S. import controls limiting Chinese textile shipments to the United States.

In the political field, however, China's approach here has been described as one of "great caution."

According to the sources, the only area where substantive discussions between deputy foreign ministers Qian and Leonid Ilyichev took place involved the problem of Soviet forces and armor along the Sino-Soviet border. It is said that some progress was made here toward thinning out the forces of both countries along their border.

In the midst of the talks, however, China formally raised another military issue on March 8 by stating objections to the possibility of transfer from the European theater to Asia of Soviet medium-range SS20 nuclear missiles. Peking's position on this issue appeared to parallel concerns voiced earlier by the Reagan administration.

In the negotiations about Afghanistan, Cambodia and Mongolia, the two sides held almost diametrically different positions. While the Russians are said to have listened politely to China's arguments on these problems, they have insisted that they involved "third countries" and as such could not be discussed in the bilateral context.

The Chinese position is that Chinese security is affected by the presence of Soviet and Vietnamese troops in the three countries. On these issues, it is understood that both sides have merely restated their positions, although some new nuances are said to have been introduced on the question of Cambodia.

The negotiations, which are due to resume in Peking within the next few months, have been shrouded in secrecy. Both Moscow and Peking are said to be interested in holding out the prospect of an improvement in their relations as a means of influencing their respective relations with the United States.