The House Agriculture Committee, threatened with a nasty floor fight, overruled one of its subcommittees yesterday and acceded to environmentalists' demands to limit extension of the basic federal pesticide law to one year.

Under the committee agreement, the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) would be extended without major change to Sept. 30, 1984, while the subcommittee would move quickly to review tightening amendments sought by the environmental groups.

The committee action yesterday virtually assured that FIFRA will go to the House floor for final approval without controversy and without possible new threats to the agriculture panel's continuing jurisdiction over the main law governing registration and use of farm and home pesticides.

The action also was an embarrassing setback for the subcommittee, headed by Rep. George E. Brown Jr. (D-Calif.), which last week went along with Reagan administration and chemical industry appeals for a simple two-year extension of FIFRA.

Warned that the two-year approach was inviting a floor battle, the committee accepted an amendment by Rep. Tom Harkin (D-Iowa) calling for a one-year extension. Harkin also is preparing a package of tightening amendments that would be reviewed by Brown's subcommittee.

Advocates of the two-year extension argued that recent turmoil at the Environmental Protection Agency had clouded enforcement issues and that administrator-designate William D. Ruckelshaus and his new management team deserved extra time to develop positions on proposed FIFRA changes.

But representatives of the Natural Resources Defense Council, the Audubon Society, the National Coalition Against the Misuse of Pesticides and other groups contended that inadequacies in EPA pesticide regulation had been documented amply by the Brown panel and others and that remedial action could not wait through another election year.

The organizations had put Agriculture Chairman E (Kika) de la Garza (D-Tex.) on notice that they would mobilize a floor assault on Brown's proposal to push the two-year extension to passage on the suspension calendar, usually reserved for noncontroversial issues.

De la Garza also was pressed by seven influential legislators, not members of his committee, who wrote that the two-year approach "will forestall timely attention to urgent legislative concerns that may underlie the severe problems at EPA....The public needs to know that the Congress is reviewing programs and taking action."

The protestors included Reps. Elliott H. Levitas (D-Ga.), Henry A. Waxman (D-Calif.) and Albert Gore Jr. (D-Tenn.).