The nations of the world should spend about $150 billion a year -- about four times the current amount -- fighting threats to the environment over the next decade, a private research group says.

The Worldwatch Institute, in its annual "State of the World" report, said major efforts are needed to restore and protect topsoil on crop land; restore forests; slow population growth; retire debts of developing countries; raise energy efficiency, and develop renewable energy sources.

"If the world stays on the cur- rent path, crises will compound and accelerate until they overwhelm

the capacity of institutions to respond," wrote Lester Brown and Edward C. Wolf in the report's summary.

"Time is of the essence: Species lost cannot be recreated. Soil washed away may take centuries, if not millennia, to replace even under careful husbandry. Once the earth gets warmed there will be no practical way of cooling it."

The institute, headed by Brown, is a research center that proposes policies it believes will advance "sustainable" societies -- those that do not jeopardize the success of future generations. Its five previous annual reports have contained similar warnings.

Some spending issues interlock, Brown and Wolf said. For example, they wrote, restoration of tropical forests not only would protect thousands of little-known species but also help absorb carbon dioxide from the atmosphere -- delaying the warming of the Earth known as the "greenhouse effect."

Stabilizing the world's forested areas could neutralize about 1.5 million tons of carbon emissions annually, about a fifth of the 1986 total, the report said.

They estimated that the United States uses twice as much energy to produce the same dollar's worth of goods or services as Japan, and the Soviet Union uses three times as much. And Japan, they said, is not running at maximum efficiency.

The report suggested varying expenditures for different purposes from year to year, with the total averaging $150 billion annually. By 1999, the breakdown would be: protecting topsoil, $24 billion; reforestation, $7 billion; slowing population growth and providing ancillary social services such as literacy training, $32 billion; increasing energy efficiency, $50 billion; developing renewable sources of energy, $27 billion, and retiring debt of developing countries, $10 billion.

Brown estimated that about $36 billion is spent worldwide for these efforts.

The report made no specific revenue proposals, except to say the money could be culled from the $900 billion a year the nations of the world now devote to military expenditures.

China is a possible example, it said. From 13 percent of gross national product a decade ago, military spending is now 7 percent of a larger economy and "investments in family planning, reforestation and food production have expanded dramatically."